<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019d403f-60a3-740d-bb20-fbc9b5db64c3,g:fec1bef8-4213-488d-a8da-1ea51110e49e-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.labcorp.com/role/CoverPage" xlink:type="simple" xlink:href="lh-20260331.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/CoverPage" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019d403f-620a-730e-a590-bc87d44a0d5b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019d403f-620a-76ad-b107-24929e6dab55" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019d403f-620a-730e-a590-bc87d44a0d5b" xlink:to="loc_dei_DocumentInformationTable_019d403f-620a-76ad-b107-24929e6dab55" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationDocumentAxis_019d403f-620a-7999-a47a-36c33eef0d13" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationDocumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019d403f-620a-76ad-b107-24929e6dab55" xlink:to="loc_dei_DocumentInformationDocumentAxis_019d403f-620a-7999-a47a-36c33eef0d13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentDomain_019d403f-620a-7999-a47a-36c33eef0d13_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_DocumentInformationDocumentAxis_019d403f-620a-7999-a47a-36c33eef0d13" xlink:to="loc_dei_DocumentDomain_019d403f-620a-7999-a47a-36c33eef0d13_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentDomain_019d403f-620a-74e7-b5bc-07632297ca44" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_DocumentInformationDocumentAxis_019d403f-620a-7999-a47a-36c33eef0d13" xlink:to="loc_dei_DocumentDomain_019d403f-620a-74e7-b5bc-07632297ca44" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUnauditedParentheticals" xlink:type="simple" xlink:href="lh-20260331.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSUnauditedParentheticals"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUnauditedParentheticals" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019d403f-620a-7626-9a71-5a2a4455613d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019d403f-620a-747b-b503-44638173f579" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7626-9a71-5a2a4455613d" xlink:to="loc_us-gaap_StatementTable_019d403f-620a-747b-b503-44638173f579" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019d403f-620a-7e82-b743-034ac4540981" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019d403f-620a-747b-b503-44638173f579" xlink:to="loc_srt_RestatementAxis_019d403f-620a-7e82-b743-034ac4540981" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019d403f-620a-7e82-b743-034ac4540981_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019d403f-620a-7e82-b743-034ac4540981" xlink:to="loc_srt_RestatementDomain_019d403f-620a-7e82-b743-034ac4540981_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019d403f-620a-7d74-b22c-a341161b39c5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019d403f-620a-7e82-b743-034ac4540981" xlink:to="loc_srt_RestatementDomain_019d403f-620a-7d74-b22c-a341161b39c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019d403f-620a-7a62-b7c8-42429f7fe3d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019d403f-620a-747b-b503-44638173f579" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019d403f-620a-7a62-b7c8-42429f7fe3d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019d403f-620a-7a62-b7c8-42429f7fe3d2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019d403f-620a-7a62-b7c8-42429f7fe3d2" xlink:to="loc_us-gaap_ClassOfStockDomain_019d403f-620a-7a62-b7c8-42429f7fe3d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019d403f-620a-74ab-b8cb-986e1121bad3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019d403f-620a-7a62-b7c8-42429f7fe3d2" xlink:to="loc_us-gaap_ClassOfStockDomain_019d403f-620a-74ab-b8cb-986e1121bad3" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEEARNINGSUnaudited" xlink:type="simple" xlink:href="lh-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEEARNINGSUnaudited"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEEARNINGSUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019d403f-620a-7929-a44c-6086c4f1d2c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_StatementTable_019d403f-620a-7929-a44c-6086c4f1d2c8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019d403f-620a-761b-b462-9fecabbbe486" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019d403f-620a-7929-a44c-6086c4f1d2c8" xlink:to="loc_srt_StatementScenarioAxis_019d403f-620a-761b-b462-9fecabbbe486" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-761b-b462-9fecabbbe486_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620a-761b-b462-9fecabbbe486" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-761b-b462-9fecabbbe486_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-7132-9c79-019fbc7970de" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620a-761b-b462-9fecabbbe486" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-7132-9c79-019fbc7970de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019d403f-620a-7508-a5e3-37ea483c024b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_ProfitLoss_019d403f-620a-7508-a5e3-37ea483c024b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax_019d403f-620a-7c06-9007-a0b13a66420f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentBeforeTax_019d403f-620a-7c06-9007-a0b13a66420f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax_019d403f-620a-7b7e-87e5-4c9abd503def" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax_019d403f-620a-7b7e-87e5-4c9abd503def" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_019d403f-620a-7a40-9430-f68384ee6eb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_019d403f-620a-7a40-9430-f68384ee6eb5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_019d403f-620a-7153-816b-033a7c85265a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_019d403f-620a-7153-816b-033a7c85265a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d403f-620a-7452-b79f-a2fa0c715674" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d403f-620a-7452-b79f-a2fa0c715674" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019d403f-620a-70cc-87e0-9c1770476617" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019d403f-620a-70cc-87e0-9c1770476617" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_019d403f-620a-777c-8e3a-c03e147691c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_019d403f-620a-777c-8e3a-c03e147691c6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_019d403f-620a-76d1-97e6-a5d83d40508f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-749b-8cf8-954a0cf4615e" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_019d403f-620a-76d1-97e6-a5d83d40508f" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUnaudited" xlink:type="simple" xlink:href="lh-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUnaudited"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019d403f-620a-74c9-8f71-dae99e77245a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_StatementTable_019d403f-620a-74c9-8f71-dae99e77245a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019d403f-620a-7240-a868-a1912261dfbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019d403f-620a-74c9-8f71-dae99e77245a" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019d403f-620a-7240-a868-a1912261dfbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-620a-7240-a868-a1912261dfbf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-620a-7240-a868-a1912261dfbf" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-620a-7240-a868-a1912261dfbf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-620a-7240-a868-a1912261dfbf" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019d403f-620a-72ed-bc38-5d484edfdfbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:to="loc_us-gaap_CommonStockMember_019d403f-620a-72ed-bc38-5d484edfdfbd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019d403f-620a-7194-a8fe-5b1d3e4134c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019d403f-620a-7194-a8fe-5b1d3e4134c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019d403f-620a-70aa-87db-84422a8a3ec7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:to="loc_us-gaap_RetainedEarningsMember_019d403f-620a-70aa-87db-84422a8a3ec7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019d403f-620a-775b-a463-a4218430cb4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019d403f-620a-75e7-91b5-a5f825953128" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019d403f-620a-775b-a463-a4218430cb4e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019d403f-620a-7f77-ba8e-26c3b59ab3d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_StockholdersEquity_019d403f-620a-7f77-ba8e-26c3b59ab3d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019d403f-620a-7cd3-9464-65b92b33fd9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_NetIncomeLoss_019d403f-620a-7cd3-9464-65b92b33fd9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d403f-620a-7546-aad6-6f846f48266d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d403f-620a-7546-aad6-6f846f48266d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Dividends_019d403f-620a-7085-810f-3399082dd883" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Dividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_Dividends_019d403f-620a-7085-810f-3399082dd883" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_Issuanceofcommonstockunderemployeeplans_019d403f-620a-73e8-a03c-4af91eafaf4c" xlink:href="lh-20260331.xsd#lh_Issuanceofcommonstockunderemployeeplans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_lh_Issuanceofcommonstockunderemployeeplans_019d403f-620a-73e8-a03c-4af91eafaf4c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019d403f-620a-7628-bb28-4dd716fc5bce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019d403f-620a-7628-bb28-4dd716fc5bce" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019d403f-620a-706e-bc78-98c244b00a0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019d403f-620a-706e-bc78-98c244b00a0c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019d403f-620a-7d63-abee-1c8b099d7a67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-764f-82ea-7fc152ce6842" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019d403f-620a-7d63-abee-1c8b099d7a67" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019d403f-620a-7d59-8529-cc388f45d805" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="simple" xlink:href="lh-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019d403f-620a-76f6-923d-e9931f14a916" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_StatementTable_019d403f-620a-76f6-923d-e9931f14a916" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019d403f-620a-7d58-84e8-31c02a6ad1ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019d403f-620a-76f6-923d-e9931f14a916" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019d403f-620a-7d58-84e8-31c02a6ad1ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019d403f-620a-7d58-84e8-31c02a6ad1ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019d403f-620a-7d58-84e8-31c02a6ad1ed" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019d403f-620a-7d58-84e8-31c02a6ad1ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019d403f-620a-726a-a159-42928aead3a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019d403f-620a-7d58-84e8-31c02a6ad1ed" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019d403f-620a-726a-a159-42928aead3a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-75eb-95aa-bada781604b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-75eb-95aa-bada781604b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019d403f-620a-7fb8-a141-3434e90b1a21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-75eb-95aa-bada781604b2" xlink:to="loc_us-gaap_ProfitLoss_019d403f-620a-7fb8-a141-3434e90b1a21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-75eb-95aa-bada781604b2" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019d403f-620a-7083-8852-4cebe3ae1013" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019d403f-620a-7083-8852-4cebe3ae1013" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019d403f-620a-7295-bd66-68a20ae9a468" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_ShareBasedCompensation_019d403f-620a-7295-bd66-68a20ae9a468" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019d403f-620a-735f-900d-28d166fb0214" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense_019d403f-620a-735f-900d-28d166fb0214" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019d403f-620a-757e-a053-2a992393aa98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019d403f-620a-757e-a053-2a992393aa98" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_019d403f-620a-7da4-b28b-5d036249b973" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherOperatingActivitiesCashFlowStatement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_019d403f-620a-7da4-b28b-5d036249b973" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-78af-a698-1e73bcf53cdc" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019d403f-620a-7d12-908e-4747e342d177" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019d403f-620a-7d12-908e-4747e342d177" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_019d403f-620a-7525-98a4-ba1ff0e673f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInContractWithCustomerAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_019d403f-620a-7525-98a4-ba1ff0e673f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInInventories_019d403f-620a-713f-a16c-57fc354a6304" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInInventories"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInInventories_019d403f-620a-713f-a16c-57fc354a6304" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019d403f-620a-79ef-8f4a-6574104e7215" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_019d403f-620a-79ef-8f4a-6574104e7215" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019d403f-620a-750b-bc68-3bf202fcc7a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019d403f-620a-750b-bc68-3bf202fcc7a2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_019d403f-620a-77aa-9c5a-714e7ac0db29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredRevenue_019d403f-620a-77aa-9c5a-714e7ac0db29" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_019d403f-620a-741e-b095-284185beb8f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019d403f-620a-75d8-b261-d0e798e2b69e" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_019d403f-620a-741e-b095-284185beb8f6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019d403f-620a-7dfd-bd2a-6cbada9279d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019d403f-620a-75eb-95aa-bada781604b2" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019d403f-620a-7dfd-bd2a-6cbada9279d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019d403f-620a-7803-a5f1-212c474ba57f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019d403f-620a-7803-a5f1-212c474ba57f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019d403f-620a-74da-a783-3064674a36dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:to="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019d403f-620a-74da-a783-3064674a36dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireInvestments_019d403f-620a-7fd0-9077-df7d28fc7062" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:to="loc_us-gaap_PaymentsToAcquireInvestments_019d403f-620a-7fd0-9077-df7d28fc7062" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019d403f-620a-7298-b0e4-a6b82d4d48ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019d403f-620a-7298-b0e4-a6b82d4d48ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019d403f-620a-798d-8f98-ee00174371d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019d403f-620a-745b-96a4-88640e9d1828" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019d403f-620a-798d-8f98-ee00174371d9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromOtherDebt_019dba27-136d-7db2-8994-53684fa2cba5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromOtherDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_ProceedsFromOtherDebt_019dba27-136d-7db2-8994-53684fa2cba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSeniorDebt_019d403f-620a-783d-a719-5fa0bba44480" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfSeniorDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_RepaymentsOfSeniorDebt_019d403f-620a-783d-a719-5fa0bba44480" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLongTermLinesOfCredit_019d403f-620a-78a4-a4d5-37b1a8908df1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromLongTermLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_ProceedsFromLongTermLinesOfCredit_019d403f-620a-78a4-a4d5-37b1a8908df1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_019d403f-620a-7b0d-95ef-56f2a04308af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_019d403f-620a-7b0d-95ef-56f2a04308af" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromAccountsReceivableSecuritization_019d403f-620a-7f63-9629-b7dac8475092" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromAccountsReceivableSecuritization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_ProceedsFromAccountsReceivableSecuritization_019d403f-620a-7f63-9629-b7dac8475092" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019d403f-620a-79c2-b781-8a20f4a13a56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019d403f-620a-79c2-b781-8a20f4a13a56" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_NetProceedsFromIssuanceOfStockToEmployees_019d403f-620a-7d82-a709-82f05da8b868" xlink:href="lh-20260331.xsd#lh_NetProceedsFromIssuanceOfStockToEmployees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_lh_NetProceedsFromIssuanceOfStockToEmployees_019d403f-620a-7d82-a709-82f05da8b868" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividends_019d403f-620a-7341-af61-b2d456e5bd07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_PaymentsOfDividends_019d403f-620a-7341-af61-b2d456e5bd07" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_019d403f-620a-756b-aaf3-6c213ef9fd69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_019d403f-620a-756b-aaf3-6c213ef9fd69" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019d403f-620a-79a0-a661-ec5d253d8f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromPaymentsForOtherFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019d403f-620a-79a0-a661-ec5d253d8f0c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019d403f-620a-7790-a4dd-4c694e46a046" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019d403f-620a-7a49-90f2-cad392f77cfe" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019d403f-620a-7790-a4dd-4c694e46a046" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019d403f-620a-74ef-8d58-89e0f47b60bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019d403f-620a-74ef-8d58-89e0f47b60bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019d403f-620a-7237-a9c0-4e6d8a650cf1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019d403f-620a-7237-a9c0-4e6d8a650cf1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019d403f-620a-7b41-9de8-5534a7045e32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019d403f-620a-7b41-9de8-5534a7045e32" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019d403f-620a-70a9-80b5-0579e4cc58aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019dd3e5-7699-706b-9a8c-d2076e064467" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019d403f-620a-7a20-ba72-dca30f17cadd" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019dd3e5-7699-706b-9a8c-d2076e064467" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/BUSINESSSEGMENTINFORMATION" xlink:type="simple" xlink:href="lh-20260331.xsd#BUSINESSSEGMENTINFORMATION"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/BUSINESSSEGMENTINFORMATION" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-7717-aac2-db4969982f62" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-7db8-a64b-1c7ab5f60b20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-7717-aac2-db4969982f62" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-7db8-a64b-1c7ab5f60b20" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7d28-8f6f-02e1ee1f2447" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-7db8-a64b-1c7ab5f60b20" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7d28-8f6f-02e1ee1f2447" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-62c5-7d28-8f6f-02e1ee1f2447_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7d28-8f6f-02e1ee1f2447" xlink:to="loc_us-gaap_SegmentDomain_019d403f-62c5-7d28-8f6f-02e1ee1f2447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-62c5-7c59-8306-3cbff1692be1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7d28-8f6f-02e1ee1f2447" xlink:to="loc_us-gaap_SegmentDomain_019d403f-62c5-7c59-8306-3cbff1692be1" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/REVENUESTables" xlink:type="simple" xlink:href="lh-20260331.xsd#REVENUESTables"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/REVENUESTables" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7e16-a95b-e31cd9a2b186" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-782a-b754-2255a742fa10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7e16-a95b-e31cd9a2b186" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-782a-b754-2255a742fa10" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7e05-ae3c-55ba556bd366" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-782a-b754-2255a742fa10" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7e05-ae3c-55ba556bd366" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7e05-ae3c-55ba556bd366_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7e05-ae3c-55ba556bd366" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7e05-ae3c-55ba556bd366_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7d63-8820-52c209c74a98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7e05-ae3c-55ba556bd366" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7d63-8820-52c209c74a98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019d403f-620a-7593-87e5-4a58e5cdd52a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-782a-b754-2255a742fa10" xlink:to="loc_srt_StatementGeographicalAxis_019d403f-620a-7593-87e5-4a58e5cdd52a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019d403f-620a-7593-87e5-4a58e5cdd52a_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019d403f-620a-7593-87e5-4a58e5cdd52a" xlink:to="loc_srt_SegmentGeographicalDomain_019d403f-620a-7593-87e5-4a58e5cdd52a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019d403f-620a-7337-a186-0196ce106d8b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019d403f-620a-7593-87e5-4a58e5cdd52a" xlink:to="loc_srt_SegmentGeographicalDomain_019d403f-620a-7337-a186-0196ce106d8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_OthercountriesMember_019d403f-620a-7479-b5f9-3895772296dd" xlink:href="lh-20260331.xsd#lh_OthercountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-7337-a186-0196ce106d8b" xlink:to="loc_lh_OthercountriesMember_019d403f-620a-7479-b5f9-3895772296dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_019d403f-620a-7f89-95b6-fedd839af332" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-7337-a186-0196ce106d8b" xlink:to="loc_srt_EuropeMember_019d403f-620a-7f89-95b6-fedd839af332" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7af7-9137-0b796ca735f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-782a-b754-2255a742fa10" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7af7-9137-0b796ca735f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-7af7-9137-0b796ca735f8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7af7-9137-0b796ca735f8" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-7af7-9137-0b796ca735f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7af7-9137-0b796ca735f8" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SelfPayMember_019d403f-620a-7936-b71a-6a6489c15ebc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SelfPayMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:to="loc_us-gaap_SelfPayMember_019d403f-620a-7936-b71a-6a6489c15ebc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ClientMember_019d403f-620a-7945-b532-c676c75dfd69" xlink:href="lh-20260331.xsd#lh_ClientMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:to="loc_lh_ClientMember_019d403f-620a-7945-b532-c676c75dfd69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_MedicareandMedicaidMember_019d403f-620a-7eea-9304-90c709eb278b" xlink:href="lh-20260331.xsd#lh_MedicareandMedicaidMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:to="loc_lh_MedicareandMedicaidMember_019d403f-620a-7eea-9304-90c709eb278b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ThirdpartyMember_019d403f-620a-789f-8ee1-68027085128f" xlink:href="lh-20260331.xsd#lh_ThirdpartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:to="loc_lh_ThirdpartyMember_019d403f-620a-789f-8ee1-68027085128f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember_019d403f-620a-7f54-90e8-4f8ee65f6430" xlink:href="lh-20260331.xsd#lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-74d6-b973-4239ed368557" xlink:to="loc_lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember_019d403f-620a-7f54-90e8-4f8ee65f6430" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTableTextBlock_019d403f-620a-7650-90df-3c274bcfa027" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7e16-a95b-e31cd9a2b186" xlink:to="loc_us-gaap_DisaggregationOfRevenueTableTextBlock_019d403f-620a-7650-90df-3c274bcfa027" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock_019d403f-620a-7ce6-8556-7a4647c04825" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7e16-a95b-e31cd9a2b186" xlink:to="loc_us-gaap_ContractWithCustomerAssetAndLiabilityTableTextBlock_019d403f-620a-7ce6-8556-7a4647c04825" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock_019d403f-620a-7140-b7d7-ea7e4f23bce7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7e16-a95b-e31cd9a2b186" xlink:to="loc_us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock_019d403f-620a-7140-b7d7-ea7e4f23bce7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYTables" xlink:type="simple" xlink:href="lh-20260331.xsd#PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYTables"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYTables" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-76e3-b3fe-1a45b4a4dd67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-7a3d-9316-a4c3de5d4c0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-76e3-b3fe-1a45b4a4dd67" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-7a3d-9316-a4c3de5d4c0c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-78cd-bd9d-e0124df6ac29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-7a3d-9316-a4c3de5d4c0c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-78cd-bd9d-e0124df6ac29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-62c5-78cd-bd9d-e0124df6ac29_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-78cd-bd9d-e0124df6ac29" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-62c5-78cd-bd9d-e0124df6ac29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-62c5-7add-aeb6-c2264f181432" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-78cd-bd9d-e0124df6ac29" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-62c5-7add-aeb6-c2264f181432" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/BASISOFFINANCIALSTATEMENTPRESENTATIONDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#BASISOFFINANCIALSTATEMENTPRESENTATIONDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/BASISOFFINANCIALSTATEMENTPRESENTATIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-723f-b52e-91a4ed0ee4da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-79f5-919a-ac09e5946a40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-723f-b52e-91a4ed0ee4da" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-79f5-919a-ac09e5946a40" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-72ae-aa98-7fabee431cca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-79f5-919a-ac09e5946a40" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-72ae-aa98-7fabee431cca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-72ae-aa98-7fabee431cca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-72ae-aa98-7fabee431cca" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-72ae-aa98-7fabee431cca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-70cb-87b4-aef1afd9790f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-72ae-aa98-7fabee431cca" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-70cb-87b4-aef1afd9790f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-620a-758f-ae7e-8765855d5a12" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-70cb-87b4-aef1afd9790f" xlink:to="loc_lh_DiagnosticsMember_019d403f-620a-758f-ae7e-8765855d5a12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7bf5-84c0-dbff85f57ef3" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-70cb-87b4-aef1afd9790f" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7bf5-84c0-dbff85f57ef3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PercentofRevenueContributed_019d403f-620a-7a55-8417-62e580ae63a9" xlink:href="lh-20260331.xsd#lh_PercentofRevenueContributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-723f-b52e-91a4ed0ee4da" xlink:to="loc_lh_PercentofRevenueContributed_019d403f-620a-7a55-8417-62e580ae63a9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/REVENUESDisaggregatedRevenueTableDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#REVENUESDisaggregatedRevenueTableDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/REVENUESDisaggregatedRevenueTableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7df7-8acb-0a44b4192bb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7df7-8acb-0a44b4192bb6" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-78a8-890f-246b1567f394" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-78a8-890f-246b1567f394" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-78a8-890f-246b1567f394_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-78a8-890f-246b1567f394" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-78a8-890f-246b1567f394_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7b64-9bb6-cf3cd1689b9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-78a8-890f-246b1567f394" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7b64-9bb6-cf3cd1689b9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-620a-78df-90a7-f7c89a964361" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7b64-9bb6-cf3cd1689b9d" xlink:to="loc_lh_DiagnosticsMember_019d403f-620a-78df-90a7-f7c89a964361" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7f9f-a077-7ef35421f31c" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7b64-9bb6-cf3cd1689b9d" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7f9f-a077-7ef35421f31c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019d403f-620a-7bf5-8a13-7bee1c64e0fc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_srt_ProductOrServiceAxis_019d403f-620a-7bf5-8a13-7bee1c64e0fc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019d403f-620a-7bf5-8a13-7bee1c64e0fc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019d403f-620a-7bf5-8a13-7bee1c64e0fc" xlink:to="loc_srt_ProductsAndServicesDomain_019d403f-620a-7bf5-8a13-7bee1c64e0fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019d403f-620a-759a-a0fb-2b6cdec28b6e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019d403f-620a-7bf5-8a13-7bee1c64e0fc" xlink:to="loc_srt_ProductsAndServicesDomain_019d403f-620a-759a-a0fb-2b6cdec28b6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerBasisOfPricingAxis_019d403f-620a-7d44-a5a4-17160d79fdfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerBasisOfPricingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_us-gaap_ContractWithCustomerBasisOfPricingAxis_019d403f-620a-7d44-a5a4-17160d79fdfd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerBasisOfPricingDomain_019d403f-620a-7d44-a5a4-17160d79fdfd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerBasisOfPricingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerBasisOfPricingAxis_019d403f-620a-7d44-a5a4-17160d79fdfd" xlink:to="loc_us-gaap_ContractWithCustomerBasisOfPricingDomain_019d403f-620a-7d44-a5a4-17160d79fdfd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerBasisOfPricingDomain_019d403f-620a-7905-8bda-bbd088f3429c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerBasisOfPricingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerBasisOfPricingAxis_019d403f-620a-7d44-a5a4-17160d79fdfd" xlink:to="loc_us-gaap_ContractWithCustomerBasisOfPricingDomain_019d403f-620a-7905-8bda-bbd088f3429c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationAxis_019d403f-620a-77b6-bc33-9dcfd21847af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_us-gaap_ContractWithCustomerDurationAxis_019d403f-620a-77b6-bc33-9dcfd21847af" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationDomain_019d403f-620a-77b6-bc33-9dcfd21847af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerDurationAxis_019d403f-620a-77b6-bc33-9dcfd21847af" xlink:to="loc_us-gaap_ContractWithCustomerDurationDomain_019d403f-620a-77b6-bc33-9dcfd21847af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerDurationDomain_019d403f-620a-747c-90b3-88c568411d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerDurationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerDurationAxis_019d403f-620a-77b6-bc33-9dcfd21847af" xlink:to="loc_us-gaap_ContractWithCustomerDurationDomain_019d403f-620a-747c-90b3-88c568411d4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019d403f-620a-7667-9863-4c2af7d7fc6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019d403f-620a-7667-9863-4c2af7d7fc6f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019d403f-620a-7667-9863-4c2af7d7fc6f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019d403f-620a-7667-9863-4c2af7d7fc6f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019d403f-620a-7667-9863-4c2af7d7fc6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019d403f-620a-7035-a24e-37fca669ce8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019d403f-620a-7667-9863-4c2af7d7fc6f" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019d403f-620a-7035-a24e-37fca669ce8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7ae4-a681-0f4abd19b610" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7ae4-a681-0f4abd19b610" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-7ae4-a681-0f4abd19b610_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7ae4-a681-0f4abd19b610" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-7ae4-a681-0f4abd19b610_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_019d403f-620a-7ae4-a681-0f4abd19b610" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ClientMember_019d403f-620a-738b-956f-5bc00709e6b0" xlink:href="lh-20260331.xsd#lh_ClientMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:to="loc_lh_ClientMember_019d403f-620a-738b-956f-5bc00709e6b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SelfPayMember_019d403f-620a-723f-88b4-7e02edb34331" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SelfPayMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:to="loc_us-gaap_SelfPayMember_019d403f-620a-723f-88b4-7e02edb34331" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_MedicareandMedicaidMember_019d403f-620a-7d58-99a2-5102c3cfaf19" xlink:href="lh-20260331.xsd#lh_MedicareandMedicaidMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:to="loc_lh_MedicareandMedicaidMember_019d403f-620a-7d58-99a2-5102c3cfaf19" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ThirdpartyMember_019d403f-620a-7f84-8882-4fd3a7863f51" xlink:href="lh-20260331.xsd#lh_ThirdpartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:to="loc_lh_ThirdpartyMember_019d403f-620a-7f84-8882-4fd3a7863f51" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember_019d403f-620a-7a5e-8e85-c14a4e881bd7" xlink:href="lh-20260331.xsd#lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_019d403f-620a-73ad-8059-6c40f198276e" xlink:to="loc_lh_PharmaceuticalBiotechnologyMedicalDeviceDiagnosticCompaniesAndCROsMember_019d403f-620a-7a5e-8e85-c14a4e881bd7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019d403f-620a-7cc4-b7d8-49193508b97e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_srt_MajorCustomersAxis_019d403f-620a-7cc4-b7d8-49193508b97e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019d403f-620a-7cc4-b7d8-49193508b97e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019d403f-620a-7cc4-b7d8-49193508b97e" xlink:to="loc_srt_NameOfMajorCustomerDomain_019d403f-620a-7cc4-b7d8-49193508b97e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019d403f-620a-70ff-ac6b-6f69dcb9ff43" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019d403f-620a-7cc4-b7d8-49193508b97e" xlink:to="loc_srt_NameOfMajorCustomerDomain_019d403f-620a-70ff-ac6b-6f69dcb9ff43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019d403f-620a-7e99-95bf-b143216b19c1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019d403f-620a-7a89-956c-a265c732e433" xlink:to="loc_srt_StatementGeographicalAxis_019d403f-620a-7e99-95bf-b143216b19c1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019d403f-620a-7e99-95bf-b143216b19c1_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019d403f-620a-7e99-95bf-b143216b19c1" xlink:to="loc_srt_SegmentGeographicalDomain_019d403f-620a-7e99-95bf-b143216b19c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019d403f-620a-7e99-95bf-b143216b19c1" xlink:to="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NorthAmericaMember_019d403f-620a-7178-8e4b-544cf2dcf40f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NorthAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:to="loc_srt_NorthAmericaMember_019d403f-620a-7178-8e4b-544cf2dcf40f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_019d403f-620a-7806-ba5c-34135027cc21" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:to="loc_srt_EuropeMember_019d403f-620a-7806-ba5c-34135027cc21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_OthercountriesMember_019d403f-620a-7807-ac34-2ebcfefd54f4" xlink:href="lh-20260331.xsd#lh_OthercountriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:to="loc_lh_OthercountriesMember_019d403f-620a-7807-ac34-2ebcfefd54f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019d403f-620a-76bd-b2ac-eb10d566c338" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019d403f-620a-796e-9ee4-5078d8ba3835" xlink:to="loc_country_US_019d403f-620a-76bd-b2ac-eb10d566c338" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PercentofRevenueContributed_019d403f-620a-7ecd-aef3-3a5ac4304df0" xlink:href="lh-20260331.xsd#lh_PercentofRevenueContributed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7df7-8acb-0a44b4192bb6" xlink:to="loc_lh_PercentofRevenueContributed_019d403f-620a-7ecd-aef3-3a5ac4304df0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019d403f-620a-75bb-8620-4fd49db0a0b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7df7-8acb-0a44b4192bb6" xlink:to="loc_us-gaap_Revenues_019d403f-620a-75bb-8620-4fd49db0a0b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PercentOfDomesticRevenue_019dd3f4-c298-750a-badc-e5c0914a8be6" xlink:href="lh-20260331.xsd#lh_PercentOfDomesticRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019d403f-620a-7df7-8acb-0a44b4192bb6" xlink:to="loc_lh_PercentOfDomesticRevenue_019dd3f4-c298-750a-badc-e5c0914a8be6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/REVENUESDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#REVENUESDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/REVENUESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d403f-620a-7433-adfd-e39fec509822" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d403f-620a-7433-adfd-e39fec509822" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7f68-a596-4f0fe064eb71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019d403f-620a-7433-adfd-e39fec509822" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7f68-a596-4f0fe064eb71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7f68-a596-4f0fe064eb71_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7f68-a596-4f0fe064eb71" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7f68-a596-4f0fe064eb71_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7418-9fb3-5a90a9dcc0a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7f68-a596-4f0fe064eb71" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7418-9fb3-5a90a9dcc0a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-620a-7b80-9cb2-14035d7ec0ab" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7418-9fb3-5a90a9dcc0a5" xlink:to="loc_lh_DiagnosticsMember_019d403f-620a-7b80-9cb2-14035d7ec0ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7872-a8e5-84c054827200" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7418-9fb3-5a90a9dcc0a5" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7872-a8e5-84c054827200" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableGross_019d403f-620a-73a6-b0cd-b3f0343f42e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_AccountsReceivableGross_019d403f-620a-73a6-b0cd-b3f0343f42e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019d403f-620a-770f-899d-feae3b89ad7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019d403f-620a-770f-899d-feae3b89ad7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019d403f-620a-7122-b006-322e94cbfbe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019d403f-620a-7122-b006-322e94cbfbe9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGross_019d403f-620a-7ab3-bb03-c423f2551df8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerAssetGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_ContractWithCustomerAssetGross_019d403f-620a-7ab3-bb03-c423f2551df8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulOtherReceivablesCurrent_019d403f-620a-70fa-aa4c-f0dc70177bdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulOtherReceivablesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_AllowanceForDoubtfulOtherReceivablesCurrent_019d403f-620a-70fa-aa4c-f0dc70177bdc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetNetCurrent_019d403f-620a-7926-973a-d29fb22705ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerAssetNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_ContractWithCustomerAssetNetCurrent_019d403f-620a-7926-973a-d29fb22705ed" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AllowanceforCreditLossWriteOff_019d403f-620a-7754-a33b-91606b29c670" xlink:href="lh-20260331.xsd#lh_AllowanceforCreditLossWriteOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_AllowanceforCreditLossWriteOff_019d403f-620a-7754-a33b-91606b29c670" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_UnbilledServicesForeignCurrencyTranslationImpact_019d403f-620a-723c-bb60-a3770ab88c56" xlink:href="lh-20260331.xsd#lh_UnbilledServicesForeignCurrencyTranslationImpact"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_UnbilledServicesForeignCurrencyTranslationImpact_019d403f-620a-723c-bb60-a3770ab88c56" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_UnbilledServicesAllowanceforCreditLoss_019d403f-620a-7df0-b5a6-7be98a42050a" xlink:href="lh-20260331.xsd#lh_UnbilledServicesAllowanceforCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_UnbilledServicesAllowanceforCreditLoss_019d403f-620a-7df0-b5a6-7be98a42050a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AllowanceForCreditLossForeignCurrencyTranslationImpact_019d403f-620a-73b3-94ed-a63191b31f76" xlink:href="lh-20260331.xsd#lh_AllowanceForCreditLossForeignCurrencyTranslationImpact"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_AllowanceForCreditLossForeignCurrencyTranslationImpact_019d403f-620a-73b3-94ed-a63191b31f76" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AllowanceforCreditLoss_019d403f-620a-7444-a01e-84a0a92fbffe" xlink:href="lh-20260331.xsd#lh_AllowanceforCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_AllowanceforCreditLoss_019d403f-620a-7444-a01e-84a0a92fbffe" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CreditLossExpenseReversal_019d403f-620a-7719-bead-6f49f25725d2" xlink:href="lh-20260331.xsd#lh_CreditLossExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_CreditLossExpenseReversal_019d403f-620a-7719-bead-6f49f25725d2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_UnbilledServicesCreditLossExpenseReversal_019d403f-620a-718a-910d-db0b69a67562" xlink:href="lh-20260331.xsd#lh_UnbilledServicesCreditLossExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_UnbilledServicesCreditLossExpenseReversal_019d403f-620a-718a-910d-db0b69a67562" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_019d403f-620a-7b81-8d89-e5c48a1caeef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_019d403f-620a-7b81-8d89-e5c48a1caeef" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_UnbilledServicesAllowanceForCreditLossWriteOff_019d403f-620a-7708-91a6-6a54efb8d58f" xlink:href="lh-20260331.xsd#lh_UnbilledServicesAllowanceForCreditLossWriteOff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_UnbilledServicesAllowanceForCreditLossWriteOff_019d403f-620a-7708-91a6-6a54efb8d58f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_019d403f-620a-73a8-b623-baa2a1d92343" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_019d403f-620a-73a8-b623-baa2a1d92343" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AccountsReceivableForeignCurrencyTranslationImpact_019d403f-620a-7007-b893-0eb9fc7f4918" xlink:href="lh-20260331.xsd#lh_AccountsReceivableForeignCurrencyTranslationImpact"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019d403f-620a-7ea4-a805-c4b2850b1876" xlink:to="loc_lh_AccountsReceivableForeignCurrencyTranslationImpact_019d403f-620a-7007-b893-0eb9fc7f4918" xlink:type="arc" order="16"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/BUSINESSACQUISITIONSDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#BUSINESSACQUISITIONSDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/BUSINESSACQUISITIONSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-71b8-8c47-f90c49e60181" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-71b8-8c47-f90c49e60181" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-71b8-8c47-f90c49e60181_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-71b8-8c47-f90c49e60181" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-71b8-8c47-f90c49e60181_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-73b6-819e-16ca29eb503c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-71b8-8c47-f90c49e60181" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-73b6-819e-16ca29eb503c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-7f9d-96c3-0cdaac526a13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-73b6-819e-16ca29eb503c" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-7f9d-96c3-0cdaac526a13" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7001-94e1-b25b27c1f12e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-73b6-819e-16ca29eb503c" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7001-94e1-b25b27c1f12e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7d47-bb78-c8fb31da100b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7d47-bb78-c8fb31da100b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7d47-bb78-c8fb31da100b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7d47-bb78-c8fb31da100b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7d47-bb78-c8fb31da100b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7d47-bb78-c8fb31da100b" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_EmpireCityLaboratoriesInc.Member_019d403f-620a-7d25-8830-db1cb1499d60" xlink:href="lh-20260331.xsd#lh_EmpireCityLaboratoriesInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:to="loc_lh_EmpireCityLaboratoriesInc.Member_019d403f-620a-7d25-8830-db1cb1499d60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_OtheracquireesMember_019d403f-620a-77f2-82e9-899ebbba6d74" xlink:href="lh-20260331.xsd#lh_OtheracquireesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:to="loc_lh_OtheracquireesMember_019d403f-620a-77f2-82e9-899ebbba6d74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_MeasurementPeriodAdjustmentsMember_019d403f-620a-76ee-9e46-756c8939c507" xlink:href="lh-20260331.xsd#lh_MeasurementPeriodAdjustmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:to="loc_lh_MeasurementPeriodAdjustmentsMember_019d403f-620a-76ee-9e46-756c8939c507" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_TotalBusinessAcquisitionsAndMeasurementPeriodAdjustmentsMember_019d403f-620a-746e-8b35-ad3ced0cb526" xlink:href="lh-20260331.xsd#lh_TotalBusinessAcquisitionsAndMeasurementPeriodAdjustmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:to="loc_lh_TotalBusinessAcquisitionsAndMeasurementPeriodAdjustmentsMember_019d403f-620a-746e-8b35-ad3ced0cb526" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ParkviewHealthSystemsInc.Member_019d403f-620a-7d7c-938b-7fe6b3366c32" xlink:href="lh-20260331.xsd#lh_ParkviewHealthSystemsInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7acd-8d06-69618455a892" xlink:to="loc_lh_ParkviewHealthSystemsInc.Member_019d403f-620a-7d7c-938b-7fe6b3366c32" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019d403f-620a-771d-950e-eb5ee2aa6b4c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_srt_RangeAxis_019d403f-620a-771d-950e-eb5ee2aa6b4c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-771d-950e-eb5ee2aa6b4c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019d403f-620a-771d-950e-eb5ee2aa6b4c" xlink:to="loc_srt_RangeMember_019d403f-620a-771d-950e-eb5ee2aa6b4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-7fd3-a58c-337aaee4269d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019d403f-620a-771d-950e-eb5ee2aa6b4c" xlink:to="loc_srt_RangeMember_019d403f-620a-7fd3-a58c-337aaee4269d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019d403f-620a-762c-8fb0-b35b5d6fbef8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620a-7fd3-a58c-337aaee4269d" xlink:to="loc_srt_MaximumMember_019d403f-620a-762c-8fb0-b35b5d6fbef8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019d403f-620a-7cc0-a403-aaedf60c8379" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620a-7fd3-a58c-337aaee4269d" xlink:to="loc_srt_MinimumMember_019d403f-620a-7cc0-a403-aaedf60c8379" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-74fb-81e5-6d7f2d5ce869" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-74fb-81e5-6d7f2d5ce869" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-74fb-81e5-6d7f2d5ce869_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-74fb-81e5-6d7f2d5ce869" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-74fb-81e5-6d7f2d5ce869_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-755f-8a3a-8231ed291063" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-74fb-81e5-6d7f2d5ce869" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-755f-8a3a-8231ed291063" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019d403f-620a-78ea-b361-1423017c96bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019d403f-620a-78ea-b361-1423017c96bc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019d403f-620a-78ea-b361-1423017c96bc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019d403f-620a-78ea-b361-1423017c96bc" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019d403f-620a-78ea-b361-1423017c96bc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019d403f-620a-7998-bb51-d14b1b458cc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019d403f-620a-78ea-b361-1423017c96bc" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019d403f-620a-7998-bb51-d14b1b458cc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019d403f-620a-7d7d-a12f-c888bd76261c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019d403f-620a-7d7d-a12f-c888bd76261c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019d403f-620a-7d7d-a12f-c888bd76261c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019d403f-620a-7d7d-a12f-c888bd76261c" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019d403f-620a-7d7d-a12f-c888bd76261c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019d403f-620a-78f5-b8fa-af17aef39dbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019d403f-620a-7d7d-a12f-c888bd76261c" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019d403f-620a-78f5-b8fa-af17aef39dbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_019d403f-620a-712b-900a-fda28a11b812" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_019d403f-620a-712b-900a-fda28a11b812" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_019d403f-620a-712b-900a-fda28a11b812_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_019d403f-620a-712b-900a-fda28a11b812" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_019d403f-620a-712b-900a-fda28a11b812_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_019d403f-620a-7f25-8921-12c7367d7f71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis_019d403f-620a-712b-900a-fda28a11b812" xlink:to="loc_us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeDomain_019d403f-620a-7f25-8921-12c7367d7f71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_019d403f-620a-7796-b12f-deeb1b7a9b23" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_srt_OwnershipAxis_019d403f-620a-7796-b12f-deeb1b7a9b23" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019d403f-620a-7796-b12f-deeb1b7a9b23_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_019d403f-620a-7796-b12f-deeb1b7a9b23" xlink:to="loc_srt_OwnershipDomain_019d403f-620a-7796-b12f-deeb1b7a9b23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019d403f-620a-7ecd-82fc-e3f9e05dc367" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_019d403f-620a-7796-b12f-deeb1b7a9b23" xlink:to="loc_srt_OwnershipDomain_019d403f-620a-7ecd-82fc-e3f9e05dc367" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CurrencyAxis_019d403f-620a-7e23-861d-6c15013e31c1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CurrencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_srt_CurrencyAxis_019d403f-620a-7e23-861d-6c15013e31c1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_currency_AllCurrenciesDomain_019d403f-620a-7e23-861d-6c15013e31c1_default" xlink:href="https://xbrl.sec.gov/currency/2026/currency-2026.xsd#currency_AllCurrenciesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CurrencyAxis_019d403f-620a-7e23-861d-6c15013e31c1" xlink:to="loc_currency_AllCurrenciesDomain_019d403f-620a-7e23-861d-6c15013e31c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_currency_AllCurrenciesDomain_019d403f-620a-729e-aff1-6586225422f9" xlink:href="https://xbrl.sec.gov/currency/2026/currency-2026.xsd#currency_AllCurrenciesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CurrencyAxis_019d403f-620a-7e23-861d-6c15013e31c1" xlink:to="loc_currency_AllCurrenciesDomain_019d403f-620a-729e-aff1-6586225422f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019d403f-620a-745a-b9e9-2dd59b0f27ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019d403f-620a-7b73-a5bd-8de6e3b0f79c" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019d403f-620a-745a-b9e9-2dd59b0f27ba" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-620a-745a-b9e9-2dd59b0f27ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d403f-620a-745a-b9e9-2dd59b0f27ba" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d403f-620a-745a-b9e9-2dd59b0f27ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-620a-7f7c-afd2-546d2f47cad0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d403f-620a-745a-b9e9-2dd59b0f27ba" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d403f-620a-7f7c-afd2-546d2f47cad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019d403f-620a-7a44-bc26-3f233a59960e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019d403f-620a-7f7c-afd2-546d2f47cad0" xlink:to="loc_us-gaap_SubsequentEventMember_019d403f-620a-7a44-bc26-3f233a59960e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019d403f-620a-72be-9c64-95ba703d0988" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_019d403f-620a-72be-9c64-95ba703d0988" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019d403f-620a-70f1-8f60-1b43a0db1d04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_019d403f-620a-70f1-8f60-1b43a0db1d04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019d403f-620a-7f0b-8d9a-4d02a1e97f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019d403f-620a-7f0b-8d9a-4d02a1e97f0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019d403f-620a-72a4-9b2d-570da767354a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019d403f-620a-72a4-9b2d-570da767354a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019d403f-620a-7c9f-b56f-9e7e51e0f645" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019d403f-620a-7c9f-b56f-9e7e51e0f645" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_019d403f-620a-7832-a709-a6690a95c974" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_019d403f-620a-7832-a709-a6690a95c974" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019d403f-620a-723f-b01b-d57927d9f915" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_019d403f-620a-723f-b01b-d57927d9f915" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019d403f-620a-7177-a21a-5277f81bedf1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_019d403f-620a-7177-a21a-5277f81bedf1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_019dba3d-778e-7a1b-a278-fc552386fb16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_019dba3d-778e-7a1b-a278-fc552386fb16" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EscrowDeposit_019d6950-de2c-7507-b2c2-3e6998c9f63a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EscrowDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_EscrowDeposit_019d6950-de2c-7507-b2c2-3e6998c9f63a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_019d403f-620a-7f3e-b8c6-ea8b91cc64d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_019d403f-620a-7f3e-b8c6-ea8b91cc64d3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019d403f-620a-7d79-b86e-a78b132c4bd1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019d403f-620a-7d79-b86e-a78b132c4bd1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019d403f-620a-7162-ae40-36a7331e71c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019d403f-620a-7162-ae40-36a7331e71c2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019d403f-620a-7388-b2a9-5ef5011c69f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019d403f-620a-7fbd-bd36-b5fb91615ebd" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_019d403f-620a-7388-b2a9-5ef5011c69f5" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_019d403f-620a-77b5-bf43-cd23d9fdf7e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_019d403f-620a-78d4-bc81-9cd91c83110d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_019d403f-620a-77b5-bf43-cd23d9fdf7e8" xlink:to="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_019d403f-620a-78d4-bc81-9cd91c83110d" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7809-9e00-68094c1077cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentsTable_019d403f-620a-78d4-bc81-9cd91c83110d" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7809-9e00-68094c1077cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7809-9e00-68094c1077cc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7497-8455-999243e7582b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_019d403f-620a-77dd-9dc9-92a4839d2663" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_019d403f-620a-77b5-bf43-cd23d9fdf7e8" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_019d403f-620a-77dd-9dc9-92a4839d2663" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_019d403f-620a-7b35-a353-7216a10c091c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionProFormaInformationNonrecurringAdjustmentLineItems_019d403f-620a-77b5-bf43-cd23d9fdf7e8" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_019d403f-620a-7b35-a353-7216a10c091c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSChangesinCarryingAmountofGoodwillDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#GOODWILLANDINTANGIBLEASSETSChangesinCarryingAmountofGoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSChangesinCarryingAmountofGoodwillDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_019d403f-620a-71e6-b090-1e2c3cc4271a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_019d403f-620a-71e6-b090-1e2c3cc4271a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-74f4-ae95-6f9443f303bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_019d403f-620a-71e6-b090-1e2c3cc4271a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-74f4-ae95-6f9443f303bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-74f4-ae95-6f9443f303bc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-74f4-ae95-6f9443f303bc" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-74f4-ae95-6f9443f303bc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-76cc-ae31-b87411a57431" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-74f4-ae95-6f9443f303bc" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-76cc-ae31-b87411a57431" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-620a-7570-97ec-b5e33dec016c" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-76cc-ae31-b87411a57431" xlink:to="loc_lh_DiagnosticsMember_019d403f-620a-7570-97ec-b5e33dec016c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7895-9786-58c80d60e0a7" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-76cc-ae31-b87411a57431" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7895-9786-58c80d60e0a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019d403f-620a-7633-924c-d8d248e38854" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:to="loc_us-gaap_Goodwill_019d403f-620a-7633-924c-d8d248e38854" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_019d403f-620a-7b9d-8833-76cdb1f5ca12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_019d403f-620a-7b9d-8833-76cdb1f5ca12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments_019d403f-620a-769d-ade2-d98cc1dac776" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:to="loc_us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments_019d403f-620a-769d-ade2-d98cc1dac776" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_GoodwillImpairmentCumulative_019d403f-620a-7100-957e-5d45da73621c" xlink:href="lh-20260331.xsd#lh_GoodwillImpairmentCumulative"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019d403f-620a-7692-8a94-3d4c7679b25c" xlink:to="loc_lh_GoodwillImpairmentCumulative_019d403f-620a-7100-957e-5d45da73621c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSComponentsofidentifiableintangibleassetsDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#GOODWILLANDINTANGIBLEASSETSComponentsofidentifiableintangibleassetsDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSComponentsofidentifiableintangibleassetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7c5b-97b9-d260245b6172" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7c5b-97b9-d260245b6172" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-77d8-9c27-d8c85141b038" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7c5b-97b9-d260245b6172" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-77d8-9c27-d8c85141b038" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-77d8-9c27-d8c85141b038_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-77d8-9c27-d8c85141b038" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-77d8-9c27-d8c85141b038_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-77d8-9c27-d8c85141b038" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-71db-97db-6a320034442b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-71db-97db-6a320034442b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7b3d-9b0d-cda840606b5d" xlink:href="lh-20260331.xsd#lh_PatentsLicensesAndTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:to="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7b3d-9b0d-cda840606b5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7e2b-9f37-859c19de6d7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7e2b-9f37-859c19de6d7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-7831-a28a-0616752558cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-7831-a28a-0616752558cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-793d-bdaf-c357fb9d60fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LicensingAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b4d-ae78-7945e03dceb9" xlink:to="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-793d-bdaf-c357fb9d60fe" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_019d403f-620a-741a-9692-781fd2f9af5c" xlink:href="lh-20260331.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7c5b-97b9-d260245b6172" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_019d403f-620a-741a-9692-781fd2f9af5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-741a-9692-781fd2f9af5c_default" xlink:href="lh-20260331.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_019d403f-620a-741a-9692-781fd2f9af5c" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-741a-9692-781fd2f9af5c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:href="lh-20260331.xsd#lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassAxis_019d403f-620a-741a-9692-781fd2f9af5c" xlink:to="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-704d-8a2b-ba9a237633a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-704d-8a2b-ba9a237633a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7357-a878-6f10ecc699b5" xlink:href="lh-20260331.xsd#lh_PatentsLicensesAndTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:to="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7357-a878-6f10ecc699b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-74d9-a1bf-5ef8a4dd4070" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-74d9-a1bf-5ef8a4dd4070" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-7197-8a81-2eb2f38aafa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-7197-8a81-2eb2f38aafa2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-7007-a1a4-d807bbd7111d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LicensingAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_ScheduleOfOtherIntangibleAssetsByMajorClassDomain_019d403f-620a-72e7-8689-595bcad9fb2a" xlink:to="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-7007-a1a4-d807bbd7111d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019d403f-620a-73ff-a3e6-e877a96269b6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7c5b-97b9-d260245b6172" xlink:to="loc_srt_RangeAxis_019d403f-620a-73ff-a3e6-e877a96269b6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-73ff-a3e6-e877a96269b6_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019d403f-620a-73ff-a3e6-e877a96269b6" xlink:to="loc_srt_RangeMember_019d403f-620a-73ff-a3e6-e877a96269b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-7ac2-a1a8-220df8deecff" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019d403f-620a-73ff-a3e6-e877a96269b6" xlink:to="loc_srt_RangeMember_019d403f-620a-7ac2-a1a8-220df8deecff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019d403f-620a-7639-b361-8fd6e5efab93" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620a-7ac2-a1a8-220df8deecff" xlink:to="loc_srt_MinimumMember_019d403f-620a-7639-b361-8fd6e5efab93" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019d403f-620a-725a-aaac-6f1e0353c54c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620a-7ac2-a1a8-220df8deecff" xlink:to="loc_srt_MaximumMember_019d403f-620a-725a-aaac-6f1e0353c54c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019d403f-620a-7c42-aa09-0afba1a1486c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019d403f-620a-7c42-aa09-0afba1a1486c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019d403f-620a-70c1-857c-b90b957892ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019d403f-620a-70c1-857c-b90b957892ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019d403f-620a-7292-ace9-0d4a5e8d93a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019d403f-620a-7292-ace9-0d4a5e8d93a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019d403f-620a-7ccb-93e8-26e6286c1cdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019d403f-620a-7ccb-93e8-26e6286c1cdb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_019d403f-620a-7921-857c-f6db7f5d2400" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_019d403f-620a-7921-857c-f6db7f5d2400" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_019d403f-620a-7682-970e-b3f0e8b99786" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_019d403f-620a-7682-970e-b3f0e8b99786" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019d403f-620a-71a8-9f47-f84b49616027" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-737a-9afb-528485897f63" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019d403f-620a-71a8-9f47-f84b49616027" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#GOODWILLANDINTANGIBLEASSETSDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/GOODWILLANDINTANGIBLEASSETSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7b53-850a-f82393fd714b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7b53-850a-f82393fd714b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b53-850a-f82393fd714b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7b53-850a-f82393fd714b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7b53-850a-f82393fd714b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7b53-850a-f82393fd714b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-787c-a513-f89840264ece" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019d403f-620a-787c-a513-f89840264ece" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7cfc-880d-6ea301014965" xlink:href="lh-20260331.xsd#lh_PatentsLicensesAndTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:to="loc_lh_PatentsLicensesAndTechnologyMember_019d403f-620a-7cfc-880d-6ea301014965" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7b1b-a65b-510d5cce9c3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019d403f-620a-7b1b-a65b-510d5cce9c3a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-72bb-9c84-6f3005128fff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_019d403f-620a-72bb-9c84-6f3005128fff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-7565-a510-ccd1c543226e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LicensingAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-72ae-a935-ad819e84f80b" xlink:to="loc_us-gaap_LicensingAgreementsMember_019d403f-620a-7565-a510-ccd1c543226e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7803-bb22-2eafba991447" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7803-bb22-2eafba991447" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7803-bb22-2eafba991447_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7803-bb22-2eafba991447" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7803-bb22-2eafba991447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-620a-7c85-a90c-a84e5485d4ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-620a-7803-bb22-2eafba991447" xlink:to="loc_us-gaap_SegmentDomain_019d403f-620a-7c85-a90c-a84e5485d4ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-620a-7f29-97dd-250e0c2d455b" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7c85-a90c-a84e5485d4ac" xlink:to="loc_lh_DiagnosticsMember_019d403f-620a-7f29-97dd-250e0c2d455b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7885-b060-d7e20f8f40d8" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-620a-7c85-a90c-a84e5485d4ac" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-620a-7885-b060-d7e20f8f40d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7489-bb76-782123a31a51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7489-bb76-782123a31a51" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7489-bb76-782123a31a51_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7489-bb76-782123a31a51" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7489-bb76-782123a31a51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7392-8e9e-0acfa60cd515" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_019d403f-620a-7489-bb76-782123a31a51" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_019d403f-620a-7392-8e9e-0acfa60cd515" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitAxis_019d403f-620a-7247-9a3d-38039f069a1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019d403f-620a-7502-be51-bf7d2745f33f" xlink:to="loc_us-gaap_ReportingUnitAxis_019d403f-620a-7247-9a3d-38039f069a1d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019d403f-620a-7247-9a3d-38039f069a1d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReportingUnitAxis_019d403f-620a-7247-9a3d-38039f069a1d" xlink:to="loc_us-gaap_ReportingUnitDomain_019d403f-620a-7247-9a3d-38039f069a1d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019d403f-620a-779a-ba88-77f3d4f049b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReportingUnitAxis_019d403f-620a-7247-9a3d-38039f069a1d" xlink:to="loc_us-gaap_ReportingUnitDomain_019d403f-620a-779a-ba88-77f3d4f049b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019d403f-620a-7605-9414-bc006e62bc9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019d403f-620a-7605-9414-bc006e62bc9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_019d403f-620a-74b1-8c1e-a8523a6447f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_019d403f-620a-74b1-8c1e-a8523a6447f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_019d403f-620a-7f96-991b-9e8ac255ae7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_019d403f-620a-7f96-991b-9e8ac255ae7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_019d403f-620a-78b2-baf8-0da986a9734a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_019d403f-620a-78b2-baf8-0da986a9734a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_019d403f-620a-7c73-8fe6-17ab926ed191" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_019d403f-620a-7c73-8fe6-17ab926ed191" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_019d403f-620a-7633-91bc-5a6c4dbeacdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_019d403f-620a-7633-91bc-5a6c4dbeacdb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive_019d403f-620a-72d0-98f1-97f37fbddda8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019d403f-620a-72c7-9988-f8b5529c6c67" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive_019d403f-620a-72d0-98f1-97f37fbddda8" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/DEBTShorttermborrowingsandcurrentportionoflongtermdebtTableDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#DEBTShorttermborrowingsandcurrentportionoflongtermdebtTableDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/DEBTShorttermborrowingsandcurrentportionoflongtermdebtTableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShortTermDebtTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7c1a-9f69-1a8cacf7316c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7c1a-9f69-1a8cacf7316c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7c1a-9f69-1a8cacf7316c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7c1a-9f69-1a8cacf7316c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7c1a-9f69-1a8cacf7316c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7e9e-a3e3-34546534d2b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7c1a-9f69-1a8cacf7316c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7e9e-a3e3-34546534d2b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ea9-a03d-bf398fc888e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:to="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ea9-a03d-bf398fc888e9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7ea9-a03d-bf398fc888e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ea9-a03d-bf398fc888e9" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7ea9-a03d-bf398fc888e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7bfd-afb7-5c696b2da223" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ea9-a03d-bf398fc888e9" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7bfd-afb7-5c696b2da223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620a-797f-8bf2-838607e9fea2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620a-797f-8bf2-838607e9fea2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620a-797f-8bf2-838607e9fea2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620a-797f-8bf2-838607e9fea2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620a-797f-8bf2-838607e9fea2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620a-7f9d-a5d1-60c73365919c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620a-797f-8bf2-838607e9fea2" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620a-7f9d-a5d1-60c73365919c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019d403f-620a-725a-b5d8-ab0133a484ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019d403f-620a-725a-b5d8-ab0133a484ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019d403f-620a-725a-b5d8-ab0133a484ab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019d403f-620a-725a-b5d8-ab0133a484ab" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019d403f-620a-725a-b5d8-ab0133a484ab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_019d403f-620a-7e1e-98dc-fbfbfba41d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_019d403f-620a-725a-b5d8-ab0133a484ab" xlink:to="loc_us-gaap_HedgingRelationshipDomain_019d403f-620a-7e1e-98dc-fbfbfba41d2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019d403f-620a-71a3-8525-5a4b597f967f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShortTermDebtTable_019d403f-620a-76e4-8d32-b59d7042f173" xlink:to="loc_us-gaap_DebtInstrumentAxis_019d403f-620a-71a3-8525-5a4b597f967f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-71a3-8525-5a4b597f967f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019d403f-620a-71a3-8525-5a4b597f967f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-71a3-8525-5a4b597f967f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-7d6f-908f-183cb05ad28b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019d403f-620a-71a3-8525-5a4b597f967f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-7d6f-908f-183cb05ad28b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CurrentPortionOfLongTermDebtMember_019d403f-620a-7a60-aae5-3e06ff093ef5" xlink:href="lh-20260331.xsd#lh_CurrentPortionOfLongTermDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-7d6f-908f-183cb05ad28b" xlink:to="loc_lh_CurrentPortionOfLongTermDebtMember_019d403f-620a-7a60-aae5-3e06ff093ef5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A1.55SeniorNotesDue2026Member_019dd3d1-56db-786c-887c-db8e7a69d8de" xlink:href="lh-20260331.xsd#lh_A1.55SeniorNotesDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620a-7d6f-908f-183cb05ad28b" xlink:to="loc_lh_A1.55SeniorNotesDue2026Member_019dd3d1-56db-786c-887c-db8e7a69d8de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A1.55SeniorNotesDue2026_019d403f-620a-70fe-bd32-acda4beb4820" xlink:href="lh-20260331.xsd#lh_A1.55SeniorNotesDue2026"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_lh_A1.55SeniorNotesDue2026_019d403f-620a-70fe-bd32-acda4beb4820" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_Shorttermdebtissuancecosts_019d403f-620a-7c84-8d94-c00eb34b7671" xlink:href="lh-20260331.xsd#lh_Shorttermdebtissuancecosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_lh_Shorttermdebtissuancecosts_019d403f-620a-7c84-8d94-c00eb34b7671" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableCurrent_019d403f-620a-78e8-9b2a-b5a92294033c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_us-gaap_NotesPayableCurrent_019d403f-620a-78e8-9b2a-b5a92294033c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019d403f-620a-7d9e-a76a-7d5c5b5585a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_us-gaap_LongTermDebtCurrent_019d403f-620a-7d9e-a76a-7d5c5b5585a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd3d1-03d1-7427-8411-d8a8be5245cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtLineItems_019d403f-620a-7959-b7b0-7f0206a34a81" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd3d1-03d1-7427-8411-d8a8be5245cc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/DEBTLongtermdebtDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#DEBTLongtermdebtDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/DEBTLongtermdebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620a-7cea-b0e1-ba6016e5a3ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620a-7cea-b0e1-ba6016e5a3ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620a-7cea-b0e1-ba6016e5a3ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620a-7cea-b0e1-ba6016e5a3ad" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620a-7cea-b0e1-ba6016e5a3ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620a-70f6-ab11-b7c31af17b83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620a-7cea-b0e1-ba6016e5a3ad" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620a-70f6-ab11-b7c31af17b83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019d403f-620a-7953-b166-e7fa702ece53" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_srt_StatementScenarioAxis_019d403f-620a-7953-b166-e7fa702ece53" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-7953-b166-e7fa702ece53_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620a-7953-b166-e7fa702ece53" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-7953-b166-e7fa702ece53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-76e0-8e11-0137982034c1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620a-7953-b166-e7fa702ece53" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620a-76e0-8e11-0137982034c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019d403f-620a-7cd1-83d9-7488d95c747e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_VariableRateAxis_019d403f-620a-7cd1-83d9-7488d95c747e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019d403f-620a-7cd1-83d9-7488d95c747e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019d403f-620a-7cd1-83d9-7488d95c747e" xlink:to="loc_us-gaap_VariableRateDomain_019d403f-620a-7cd1-83d9-7488d95c747e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019d403f-620a-7c07-b861-6b21d96f3e81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019d403f-620a-7cd1-83d9-7488d95c747e" xlink:to="loc_us-gaap_VariableRateDomain_019d403f-620a-7c07-b861-6b21d96f3e81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019d403f-620a-7ea9-b754-17e47d78b8e5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_srt_RangeAxis_019d403f-620a-7ea9-b754-17e47d78b8e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-7ea9-b754-17e47d78b8e5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019d403f-620a-7ea9-b754-17e47d78b8e5" xlink:to="loc_srt_RangeMember_019d403f-620a-7ea9-b754-17e47d78b8e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620a-70e0-9ff7-1bac5b37c00a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019d403f-620a-7ea9-b754-17e47d78b8e5" xlink:to="loc_srt_RangeMember_019d403f-620a-70e0-9ff7-1bac5b37c00a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ceb-9fc8-212096724836" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ceb-9fc8-212096724836" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7ceb-9fc8-212096724836_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ceb-9fc8-212096724836" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7ceb-9fc8-212096724836_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7821-b273-8034638bd0b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620a-7ceb-9fc8-212096724836" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7821-b273-8034638bd0b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620a-711b-8d5b-4af794c6f28e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7821-b273-8034638bd0b6" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620a-711b-8d5b-4af794c6f28e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AccountsReceivableSecuritizaitonMember_019d403f-620a-7e4f-8293-0274feac547c" xlink:href="lh-20260331.xsd#lh_AccountsReceivableSecuritizaitonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019d403f-620a-7821-b273-8034638bd0b6" xlink:to="loc_lh_AccountsReceivableSecuritizaitonMember_019d403f-620a-7e4f-8293-0274feac547c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7405-aa09-81af21ca3ee7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7405-aa09-81af21ca3ee7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7405-aa09-81af21ca3ee7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7405-aa09-81af21ca3ee7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-7405-aa09-81af21ca3ee7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-708e-8bf7-2ffa064a2a27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-620a-7405-aa09-81af21ca3ee7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-620a-708e-8bf7-2ffa064a2a27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7514-bbb2-bbdeb2c2fb1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7514-bbb2-bbdeb2c2fb1e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7514-bbb2-bbdeb2c2fb1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7514-bbb2-bbdeb2c2fb1e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7514-bbb2-bbdeb2c2fb1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7046-8a43-310c867b1c75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7514-bbb2-bbdeb2c2fb1e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7046-8a43-310c867b1c75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7460-a859-21fc1a84cb39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7046-8a43-310c867b1c75" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7460-a859-21fc1a84cb39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019dde1b-aa8f-76fc-b996-3bb7f56f07d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019d403f-620a-7f36-adcd-1530ac734e6e" xlink:to="loc_us-gaap_DebtInstrumentAxis_019dde1b-aa8f-76fc-b996-3bb7f56f07d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019dde1b-aa8f-76fc-b996-3bb7f56f07d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019dde1b-aa8f-76fc-b996-3bb7f56f07d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019dde1b-aa8f-76fc-b996-3bb7f56f07d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019dde1b-aa8f-76fc-b996-3bb7f56f07d9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A3.60SeniorNotesDue2027Member_019dd3da-50e3-7533-af8c-ae504606effe" xlink:href="lh-20260331.xsd#lh_A3.60SeniorNotesDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A3.60SeniorNotesDue2027Member_019dd3da-50e3-7533-af8c-ae504606effe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2.95SeniorNotesDue2029Member_019dd3cd-a604-78b6-82d4-43cccbd14761" xlink:href="lh-20260331.xsd#lh_A2.95SeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A2.95SeniorNotesDue2029Member_019dd3cd-a604-78b6-82d4-43cccbd14761" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.35SeniorNotesDue2030Member_019dd3cd-f679-7b2a-94c5-ab2ca80dd349" xlink:href="lh-20260331.xsd#lh_A4.35SeniorNotesDue2030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A4.35SeniorNotesDue2030Member_019dd3cd-f679-7b2a-94c5-ab2ca80dd349" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2.70SeniorNotesDue2031Member_019dd3ce-4a86-7099-9599-2ffc6554f6d3" xlink:href="lh-20260331.xsd#lh_A2.70SeniorNotesDue2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A2.70SeniorNotesDue2031Member_019dd3ce-4a86-7099-9599-2ffc6554f6d3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.55SeniorNotesDue2032Member_019dd3ce-9b3e-781a-ab2a-a26c24e38a96" xlink:href="lh-20260331.xsd#lh_A4.55SeniorNotesDue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A4.55SeniorNotesDue2032Member_019dd3ce-9b3e-781a-ab2a-a26c24e38a96" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.80SeniorNotesDue2034Member_019dd3ce-edb5-776b-816e-71a9e4c2ae0c" xlink:href="lh-20260331.xsd#lh_A4.80SeniorNotesDue2034Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A4.80SeniorNotesDue2034Member_019dd3ce-edb5-776b-816e-71a9e4c2ae0c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.70SeniorNotesDue2045Member_019dd3cf-412d-7940-8261-87fa98efc2b2" xlink:href="lh-20260331.xsd#lh_A4.70SeniorNotesDue2045Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dde1c-b9ec-7e61-886f-5a5924173f4b" xlink:to="loc_lh_A4.70SeniorNotesDue2045Member_019dd3cf-412d-7940-8261-87fa98efc2b2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A1.55SeniorNotesDue2026_019d403f-620b-7276-b148-fea73ab1b052" xlink:href="lh-20260331.xsd#lh_A1.55SeniorNotesDue2026"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A1.55SeniorNotesDue2026_019d403f-620b-7276-b148-fea73ab1b052" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_TermLoanIssuedInMarch2026_019dde21-76a6-7b09-bfa8-a5516fccefdd" xlink:href="lh-20260331.xsd#lh_TermLoanIssuedInMarch2026"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_TermLoanIssuedInMarch2026_019dde21-76a6-7b09-bfa8-a5516fccefdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A360SeniorNotesDue2027_019d403f-620b-7d51-918e-870c65d8c605" xlink:href="lh-20260331.xsd#lh_A360SeniorNotesDue2027"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A360SeniorNotesDue2027_019d403f-620b-7d51-918e-870c65d8c605" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2.95seniornotesdue2029_019d403f-620b-753d-9be0-4c6eeecfe55b" xlink:href="lh-20260331.xsd#lh_A2.95seniornotesdue2029"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A2.95seniornotesdue2029_019d403f-620b-753d-9be0-4c6eeecfe55b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.35SeniorNotesDue2030_019d403f-620b-7d54-894a-f4e78976a88f" xlink:href="lh-20260331.xsd#lh_A4.35SeniorNotesDue2030"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A4.35SeniorNotesDue2030_019d403f-620b-7d54-894a-f4e78976a88f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A270SeniorNotesDue2031_019d403f-620b-741d-ac8f-1773682ce4e5" xlink:href="lh-20260331.xsd#lh_A270SeniorNotesDue2031"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A270SeniorNotesDue2031_019d403f-620b-741d-ac8f-1773682ce4e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.55SeniorNotesDue2032_019d403f-620b-78b4-a7ce-99fc0be984dd" xlink:href="lh-20260331.xsd#lh_A4.55SeniorNotesDue2032"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A4.55SeniorNotesDue2032_019d403f-620b-78b4-a7ce-99fc0be984dd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A4.80SeniorNotesDue2034_019d403f-620b-7886-a708-656217944fe0" xlink:href="lh-20260331.xsd#lh_A4.80SeniorNotesDue2034"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A4.80SeniorNotesDue2034_019d403f-620b-7886-a708-656217944fe0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A470SeniorNotesDue2045_019d403f-620b-7188-ae24-aa6b1fe78580" xlink:href="lh-20260331.xsd#lh_A470SeniorNotesDue2045"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_A470SeniorNotesDue2045_019d403f-620b-7188-ae24-aa6b1fe78580" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_Longtermdebtissuancecosts_019d403f-620b-7158-aa45-ac2c50828c06" xlink:href="lh-20260331.xsd#lh_Longtermdebtissuancecosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_lh_Longtermdebtissuancecosts_019d403f-620b-7158-aa45-ac2c50828c06" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermLineOfCredit_019d403f-620b-7000-a2f0-e009dd019821" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermLineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_us-gaap_LongTermLineOfCredit_019d403f-620b-7000-a2f0-e009dd019821" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019d403f-620b-7df5-b3b7-432a80615f28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019d403f-620b-7df5-b3b7-432a80615f28" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dde1d-6e48-71eb-ba76-c7e2da200ff5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019d403f-620b-7fb6-901e-ecf47c46b606" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dde1d-6e48-71eb-ba76-c7e2da200ff5" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/DEBTCreditFacilitiesDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#DEBTCreditFacilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/DEBTCreditFacilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019d403f-620b-7e39-9d78-91040c2650d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_DebtInstrumentAxis_019d403f-620b-7e39-9d78-91040c2650d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620b-7e39-9d78-91040c2650d5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019d403f-620b-7e39-9d78-91040c2650d5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620b-7e39-9d78-91040c2650d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620b-7023-8299-6a15043c35db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019d403f-620b-7e39-9d78-91040c2650d5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019d403f-620b-7023-8299-6a15043c35db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019d403f-620b-7572-a1f6-c4e905267be8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_CreditFacilityAxis_019d403f-620b-7572-a1f6-c4e905267be8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620b-7572-a1f6-c4e905267be8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620b-7572-a1f6-c4e905267be8" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620b-7572-a1f6-c4e905267be8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-620b-74ca-b1ed-7dab64ea0696" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-620b-7572-a1f6-c4e905267be8" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-620b-74ca-b1ed-7dab64ea0696" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7997-8808-9b1b6cf815e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019d403f-620b-74ca-b1ed-7dab64ea0696" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7997-8808-9b1b6cf815e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_AccountsReceivableSecuritizaitonMember_019d403f-620b-7ff6-825f-8c55d28dc290" xlink:href="lh-20260331.xsd#lh_AccountsReceivableSecuritizaitonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019d403f-620b-74ca-b1ed-7dab64ea0696" xlink:to="loc_lh_AccountsReceivableSecuritizaitonMember_019d403f-620b-7ff6-825f-8c55d28dc290" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2026TermLoanMember_019d403f-620b-75df-ab75-44586ed4a507" xlink:href="lh-20260331.xsd#lh_A2026TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019d403f-620b-74ca-b1ed-7dab64ea0696" xlink:to="loc_lh_A2026TermLoanMember_019d403f-620b-75df-ab75-44586ed4a507" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620b-7f75-a8de-aef097880602" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620b-7f75-a8de-aef097880602" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620b-7f75-a8de-aef097880602_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620b-7f75-a8de-aef097880602" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620b-7f75-a8de-aef097880602_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620b-7500-8875-d946f4703ab7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019d403f-620b-7f75-a8de-aef097880602" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620b-7500-8875-d946f4703ab7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7e9f-b1db-ed3e8357ddf5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019d403f-620b-7500-8875-d946f4703ab7" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7e9f-b1db-ed3e8357ddf5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7a53-85f3-3d4f1c3a83f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7a53-85f3-3d4f1c3a83f5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7a53-85f3-3d4f1c3a83f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7a53-85f3-3d4f1c3a83f5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7a53-85f3-3d4f1c3a83f5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-620b-7a53-85f3-3d4f1c3a83f5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A360SeniorNotesDue2027_019d403f-620b-77b4-aec6-2b21d57a73e2" xlink:href="lh-20260331.xsd#lh_A360SeniorNotesDue2027"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:to="loc_lh_A360SeniorNotesDue2027_019d403f-620b-77b4-aec6-2b21d57a73e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2.95seniornotesdue2029_019d403f-620b-79e9-b292-e29736bbcae9" xlink:href="lh-20260331.xsd#lh_A2.95seniornotesdue2029"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:to="loc_lh_A2.95seniornotesdue2029_019d403f-620b-79e9-b292-e29736bbcae9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A270SeniorNotesDue2031_019d403f-620b-7ed5-bd36-dcf68221677e" xlink:href="lh-20260331.xsd#lh_A270SeniorNotesDue2031"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:to="loc_lh_A270SeniorNotesDue2031_019d403f-620b-7ed5-bd36-dcf68221677e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A470SeniorNotesDue2045_019d403f-620b-7ac6-b483-fa345f19a062" xlink:href="lh-20260331.xsd#lh_A470SeniorNotesDue2045"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-620b-7b60-9d8d-5a0bedf9fd45" xlink:to="loc_lh_A470SeniorNotesDue2045_019d403f-620b-7ac6-b483-fa345f19a062" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019d403f-620b-75c9-9d33-dc987a88f8e0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_srt_StatementScenarioAxis_019d403f-620b-75c9-9d33-dc987a88f8e0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620b-75c9-9d33-dc987a88f8e0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620b-75c9-9d33-dc987a88f8e0" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620b-75c9-9d33-dc987a88f8e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019d403f-620b-7926-a8b8-b436ae8e7b31" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019d403f-620b-75c9-9d33-dc987a88f8e0" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019d403f-620b-7926-a8b8-b436ae8e7b31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620b-774b-9d49-ef0a435adb2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620b-774b-9d49-ef0a435adb2c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620b-774b-9d49-ef0a435adb2c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620b-774b-9d49-ef0a435adb2c" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620b-774b-9d49-ef0a435adb2c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620b-7f1d-ab37-a5ef42d60df6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-620b-774b-9d49-ef0a435adb2c" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620b-7f1d-ab37-a5ef42d60df6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7d73-908d-85a6bb98e5f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-620b-7f1d-ab37-a5ef42d60df6" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019d403f-620b-7d73-908d-85a6bb98e5f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019d403f-620b-75f3-93d5-e460dd8055e3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_srt_RangeAxis_019d403f-620b-75f3-93d5-e460dd8055e3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620b-75f3-93d5-e460dd8055e3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019d403f-620b-75f3-93d5-e460dd8055e3" xlink:to="loc_srt_RangeMember_019d403f-620b-75f3-93d5-e460dd8055e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-620b-71d8-918e-707b2540f50e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019d403f-620b-75f3-93d5-e460dd8055e3" xlink:to="loc_srt_RangeMember_019d403f-620b-71d8-918e-707b2540f50e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019d403f-620b-7ce2-b817-3bf244663237" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620b-71d8-918e-707b2540f50e" xlink:to="loc_srt_MinimumMember_019d403f-620b-7ce2-b817-3bf244663237" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019d403f-620b-7e78-9b2c-dc43cb255cce" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019d403f-620b-71d8-918e-707b2540f50e" xlink:to="loc_srt_MaximumMember_019d403f-620b-7e78-9b2c-dc43cb255cce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019d403f-620b-7f58-af09-de41150bf4cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_019d403f-620b-756e-9914-97dba275196d" xlink:to="loc_us-gaap_VariableRateAxis_019d403f-620b-7f58-af09-de41150bf4cb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019d403f-620b-7f58-af09-de41150bf4cb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019d403f-620b-7f58-af09-de41150bf4cb" xlink:to="loc_us-gaap_VariableRateDomain_019d403f-620b-7f58-af09-de41150bf4cb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019d403f-620b-7e0e-a82f-eb72425d43c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019d403f-620b-7f58-af09-de41150bf4cb" xlink:to="loc_us-gaap_VariableRateDomain_019d403f-620b-7e0e-a82f-eb72425d43c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019d403f-620b-76f5-934d-6e5931ba69b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019d403f-620b-7e0e-a82f-eb72425d43c5" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019d403f-620b-76f5-934d-6e5931ba69b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019d403f-620b-7889-a463-5c2e74bf68af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019d403f-620b-7e0e-a82f-eb72425d43c5" xlink:to="loc_us-gaap_BaseRateMember_019d403f-620b-7889-a463-5c2e74bf68af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019d403f-620b-7b8b-93d3-56d735ae7672" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019d403f-620b-7b8b-93d3-56d735ae7672" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CreditFacilityOptiontoIncrease_019d403f-620b-7209-b80f-7a4fb4603469" xlink:href="lh-20260331.xsd#lh_CreditFacilityOptiontoIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_lh_CreditFacilityOptiontoIncrease_019d403f-620b-7209-b80f-7a4fb4603469" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CreditFacilityMaximumSwingLineBorrowings_019d403f-620b-7c8c-906e-6c1d68e257bb" xlink:href="lh-20260331.xsd#lh_CreditFacilityMaximumSwingLineBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_lh_CreditFacilityMaximumSwingLineBorrowings_019d403f-620b-7c8c-906e-6c1d68e257bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CreditFacilityMaximumLettersofCredit_019d403f-620b-7322-9929-1aac1fd4bf3d" xlink:href="lh-20260331.xsd#lh_CreditFacilityMaximumLettersofCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_lh_CreditFacilityMaximumLettersofCredit_019d403f-620b-7322-9929-1aac1fd4bf3d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019d403f-620b-7c69-a67f-6d744893f2dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019d403f-620b-7c69-a67f-6d744893f2dd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_019d403f-620b-76ad-821d-45037edbcdc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeDescription"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeeDescription_019d403f-620b-76ad-821d-45037edbcdc0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_RevolvingCreditFacilityAmountOutstanding_019d403f-620b-7a7e-b31a-72f5523ac8a4" xlink:href="lh-20260331.xsd#lh_RevolvingCreditFacilityAmountOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_lh_RevolvingCreditFacilityAmountOutstanding_019d403f-620b-7a7e-b31a-72f5523ac8a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019d403f-620b-7c73-afbe-b6152ee1173d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019d403f-620b-7c73-afbe-b6152ee1173d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_019d403f-620b-7cc2-a27f-eaf12aa9c6a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd_019d403f-620b-7cc2-a27f-eaf12aa9c6a3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_019d403f-620b-7b60-a2a1-171666f45ccf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity_019d403f-620b-7b60-a2a1-171666f45ccf" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromAccountsReceivableSecuritization_019d403f-620b-731d-879f-274cddf3d128" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromAccountsReceivableSecuritization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_ProceedsFromAccountsReceivableSecuritization_019d403f-620b-731d-879f-274cddf3d128" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_TermLoanIssuedInMarch2026_019d403f-620b-72f2-86ca-ec02c545e423" xlink:href="lh-20260331.xsd#lh_TermLoanIssuedInMarch2026"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_lh_TermLoanIssuedInMarch2026_019d403f-620b-72f2-86ca-ec02c545e423" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019d403f-620b-781b-8cec-f04b4a2d579d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019d403f-620b-781b-8cec-f04b4a2d579d" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd3d2-50d4-78ac-8fc6-358536a0f915" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019d403f-620b-7f22-9ba3-ffc7e0ccd874" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd3d2-50d4-78ac-8fc6-358536a0f915" xlink:type="arc" order="13"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019d403f-62c5-7f5e-8535-495673e0d57f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:to="loc_us-gaap_CreditFacilityAxis_019d403f-62c5-7f5e-8535-495673e0d57f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-62c5-7f5e-8535-495673e0d57f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-62c5-7f5e-8535-495673e0d57f" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-62c5-7f5e-8535-495673e0d57f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019d403f-62c5-70c2-a0a6-aef2a602417b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019d403f-62c5-7f5e-8535-495673e0d57f" xlink:to="loc_us-gaap_CreditFacilityDomain_019d403f-62c5-70c2-a0a6-aef2a602417b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019d403f-62c5-7fec-a507-7e3a83586ced" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019d403f-62c5-7fec-a507-7e3a83586ced" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019d403f-62c5-7fec-a507-7e3a83586ced_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019d403f-62c5-7fec-a507-7e3a83586ced" xlink:to="loc_us-gaap_ClassOfStockDomain_019d403f-62c5-7fec-a507-7e3a83586ced_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019d403f-62c5-7fd2-acc7-bd7bdf30c7f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019d403f-62c5-7fec-a507-7e3a83586ced" xlink:to="loc_us-gaap_ClassOfStockDomain_019d403f-62c5-7fd2-acc7-bd7bdf30c7f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019d403f-62c5-71cd-9275-7cc1e8220008" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019d403f-62c5-7fd2-acc7-bd7bdf30c7f5" xlink:to="loc_us-gaap_CommonStockMember_019d403f-62c5-71cd-9275-7cc1e8220008" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019d403f-62c5-7b8f-8bf9-f534ba24e223" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019d403f-62c5-7b8f-8bf9-f534ba24e223" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-7b8f-8bf9-f534ba24e223_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d403f-62c5-7b8f-8bf9-f534ba24e223" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-7b8f-8bf9-f534ba24e223_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-7001-b3fe-5e24a37e0823" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019d403f-62c5-7b8f-8bf9-f534ba24e223" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-7001-b3fe-5e24a37e0823" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019d403f-62c5-75e3-b1d6-e272c6e2d94f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-7001-b3fe-5e24a37e0823" xlink:to="loc_us-gaap_SubsequentEventMember_019d403f-62c5-75e3-b1d6-e272c6e2d94f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019d403f-62c5-77f3-875c-24a352ccce19" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-77f3-875c-24a352ccce19_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019d403f-62c5-78a0-b4bf-fef535004681" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsAxis_019d403f-62c5-7a02-b6a2-1f7edcd5f35b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019d403f-62c5-7baa-bd5e-4ca6d31fdde4" xlink:to="loc_us-gaap_DividendsAxis_019d403f-62c5-7a02-b6a2-1f7edcd5f35b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019d403f-62c5-7a02-b6a2-1f7edcd5f35b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DividendsAxis_019d403f-62c5-7a02-b6a2-1f7edcd5f35b" xlink:to="loc_us-gaap_DividendsDomain_019d403f-62c5-7a02-b6a2-1f7edcd5f35b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019d403f-62c5-7eba-a9ed-c40ba1501e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DividendsAxis_019d403f-62c5-7a02-b6a2-1f7edcd5f35b" xlink:to="loc_us-gaap_DividendsDomain_019d403f-62c5-7eba-a9ed-c40ba1501e9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_O2026Q2DividendsMember_019d403f-62c5-7881-bb1d-ba6508347e0b" xlink:href="lh-20260331.xsd#lh_O2026Q2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019d403f-62c5-7eba-a9ed-c40ba1501e9f" xlink:to="loc_lh_O2026Q2DividendsMember_019d403f-62c5-7881-bb1d-ba6508347e0b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CommonSharesIssuedRollforwardAbstract_019d403f-62c5-784a-abee-9745747e641a" xlink:href="lh-20260331.xsd#lh_CommonSharesIssuedRollforwardAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_lh_CommonSharesIssuedRollforwardAbstract_019d403f-62c5-784a-abee-9745747e641a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019d403f-62c5-7219-a9f2-c41e5f84a59e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_CommonSharesIssuedRollforwardAbstract_019d403f-62c5-784a-abee-9745747e641a" xlink:to="loc_us-gaap_CommonStockSharesIssued_019d403f-62c5-7219-a9f2-c41e5f84a59e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_CommonsStockIssuedDuringPeriodSharesEmployeeStockPlans_019d403f-62c5-78c3-927c-69ea979dee26" xlink:href="lh-20260331.xsd#lh_CommonsStockIssuedDuringPeriodSharesEmployeeStockPlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_CommonSharesIssuedRollforwardAbstract_019d403f-62c5-784a-abee-9745747e641a" xlink:to="loc_lh_CommonsStockIssuedDuringPeriodSharesEmployeeStockPlans_019d403f-62c5-78c3-927c-69ea979dee26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019d403f-62c5-7a36-8d34-cb7a1638500c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019d403f-62c5-72e4-9cda-8ae4598771f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019d403f-62c5-72e4-9cda-8ae4598771f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019d403f-62c5-7037-bcb7-c01904d4abe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019d403f-62c5-7037-bcb7-c01904d4abe9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019d403f-62c5-74ad-b68f-ed8ddadfe6b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019d403f-62c5-74ad-b68f-ed8ddadfe6b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019d403f-62c5-74bf-9088-8eeb23c68e54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019d403f-62c5-74bf-9088-8eeb23c68e54" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019d403f-62c5-7620-8b81-0074785df10c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019d403f-62c5-7620-8b81-0074785df10c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019d403f-62c5-7f65-9b50-039c6ef2e310" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019d403f-62c5-7f65-9b50-039c6ef2e310" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableDateDeclaredDayMonthAndYear_019d403f-62c5-79e4-9637-4f4521db5e5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsPayableDateDeclaredDayMonthAndYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_DividendsPayableDateDeclaredDayMonthAndYear_019d403f-62c5-79e4-9637-4f4521db5e5b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableAmountPerShare_019d403f-62c5-70b9-8069-58065ed7a74e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsPayableAmountPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_DividendsPayableAmountPerShare_019d403f-62c5-70b9-8069-58065ed7a74e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Dividends_019d403f-62c5-7a0b-ab2e-b1f56fa4d8e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Dividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_Dividends_019d403f-62c5-7a0b-ab2e-b1f56fa4d8e1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendPayableDateToBePaidDayMonthAndYear_019d403f-62c5-75d4-b633-ebd7a8b444a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendPayableDateToBePaidDayMonthAndYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_DividendPayableDateToBePaidDayMonthAndYear_019d403f-62c5-75d4-b633-ebd7a8b444a7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableDateOfRecordDayMonthAndYear_019d403f-62c5-7f18-b195-2556f41b4dbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsPayableDateOfRecordDayMonthAndYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019d403f-62c5-7381-b137-90440f8d783e" xlink:to="loc_us-gaap_DividendsPayableDateOfRecordDayMonthAndYear_019d403f-62c5-7f18-b195-2556f41b4dbe" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYPREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYAccumulatedOtherComprehensiveEarningsDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYPREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYAccumulatedOtherComprehensiveEarningsDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/PREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYPREFERREDSTOCKANDCOMMONSHAREHOLDERSEQUITYAccumulatedOtherComprehensiveEarningsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-777f-a6e5-a78cb10e3dbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-777f-a6e5-a78cb10e3dbd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-74b1-b19e-57929f163b10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-777f-a6e5-a78cb10e3dbd" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-74b1-b19e-57929f163b10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-74b1-b19e-57929f163b10_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-74b1-b19e-57929f163b10" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-74b1-b19e-57929f163b10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7138-bc91-6bb229a24f42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-74b1-b19e-57929f163b10" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7138-bc91-6bb229a24f42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-7052-8ec8-7a3a80a0cf25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_019d403f-62c5-777f-a6e5-a78cb10e3dbd" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-7052-8ec8-7a3a80a0cf25" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-62c5-7052-8ec8-7a3a80a0cf25_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-7052-8ec8-7a3a80a0cf25" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-62c5-7052-8ec8-7a3a80a0cf25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019d403f-62c5-714b-81da-4a7c8843a7e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019d403f-62c5-7052-8ec8-7a3a80a0cf25" xlink:to="loc_us-gaap_EquityComponentDomain_019d403f-62c5-714b-81da-4a7c8843a7e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019d403f-62c5-7f76-8b90-b34786f3b658" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019d403f-62c5-714b-81da-4a7c8843a7e1" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019d403f-62c5-7f76-8b90-b34786f3b658" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax_019d403f-62c5-7568-8014-fe7271bd5542" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax_019d403f-62c5-7568-8014-fe7271bd5542" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax_019d403f-62c5-75d8-bcff-f73ebdb5b274" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossBeforeReclassificationAndTax_019d403f-62c5-75d8-bcff-f73ebdb5b274" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax_019d403f-62c5-7141-89ba-cc5c75e8fc2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax_019d403f-62c5-7141-89ba-cc5c75e8fc2a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_019d403f-62c5-767e-841e-62c8346cb7b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax_019d403f-62c5-767e-841e-62c8346cb7b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019d403f-62c5-7420-8f5a-d99c427d2eb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019d403f-62c5-7420-8f5a-d99c427d2eb2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax_019d403f-62c5-72f6-a1aa-54841912be5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeReclassificationAdjustmentsAndTax_019d403f-62c5-72f6-a1aa-54841912be5f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_019d403f-62c5-75f5-ba3f-7b278ecac8be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_019d403f-62c5-75f5-ba3f-7b278ecac8be" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax_019d403f-62c5-7e85-86c7-987696498753" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax_019d403f-62c5-7e85-86c7-987696498753" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_019d403f-62c5-7135-9d76-262fa79ed29c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_019d403f-62c5-724a-a3fc-0a5fcce037a0" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_019d403f-62c5-7135-9d76-262fa79ed29c" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#COMMITMENTSANDCONTINGENCIESDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019d403f-62c5-70a8-8bfa-c86f36312a8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019d403f-62c5-70a8-8bfa-c86f36312a8a" xlink:to="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019d403f-62c5-77f5-b32a-20201a89a978" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019d403f-62c5-77f5-b32a-20201a89a978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019d403f-62c5-77f5-b32a-20201a89a978_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019d403f-62c5-77f5-b32a-20201a89a978" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019d403f-62c5-77f5-b32a-20201a89a978_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019d403f-62c5-7456-bee3-18dc66bd50a9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019d403f-62c5-77f5-b32a-20201a89a978" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019d403f-62c5-7456-bee3-18dc66bd50a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-7f31-8d9b-f2c5346166cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-7f31-8d9b-f2c5346166cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7f31-8d9b-f2c5346166cf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-7f31-8d9b-f2c5346166cf" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7f31-8d9b-f2c5346166cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7a0f-925a-e2c0e05383fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019d403f-62c5-7f31-8d9b-f2c5346166cf" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019d403f-62c5-7a0f-925a-e2c0e05383fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019d403f-62c5-7c4e-b7b5-ee4722a7d30d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:to="loc_srt_RangeAxis_019d403f-62c5-7c4e-b7b5-ee4722a7d30d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-62c5-7c4e-b7b5-ee4722a7d30d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019d403f-62c5-7c4e-b7b5-ee4722a7d30d" xlink:to="loc_srt_RangeMember_019d403f-62c5-7c4e-b7b5-ee4722a7d30d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019d403f-62c5-75c2-9fc8-ea1b09ee71a9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019d403f-62c5-7c4e-b7b5-ee4722a7d30d" xlink:to="loc_srt_RangeMember_019d403f-62c5-75c2-9fc8-ea1b09ee71a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019d403f-62c5-7229-a7e6-d4895c65657c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:to="loc_dei_LegalEntityAxis_019d403f-62c5-7229-a7e6-d4895c65657c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019d403f-62c5-7229-a7e6-d4895c65657c_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019d403f-62c5-7229-a7e6-d4895c65657c" xlink:to="loc_dei_EntityDomain_019d403f-62c5-7229-a7e6-d4895c65657c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019d403f-62c5-7e8f-8130-a738d78df270" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019d403f-62c5-7229-a7e6-d4895c65657c" xlink:to="loc_dei_EntityDomain_019d403f-62c5-7e8f-8130-a738d78df270" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_LossContingencyDamagesTypeAxis_019d403f-62c5-75a4-a697-a666d09688dc" xlink:href="lh-20260331.xsd#lh_LossContingencyDamagesTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019d403f-62c5-7d79-bb3b-88020fda2179" xlink:to="loc_lh_LossContingencyDamagesTypeAxis_019d403f-62c5-75a4-a697-a666d09688dc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-75a4-a697-a666d09688dc_default" xlink:href="lh-20260331.xsd#lh_LossContingencyDamagesTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lh_LossContingencyDamagesTypeAxis_019d403f-62c5-75a4-a697-a666d09688dc" xlink:to="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-75a4-a697-a666d09688dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:href="lh-20260331.xsd#lh_LossContingencyDamagesTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lh_LossContingencyDamagesTypeAxis_019d403f-62c5-75a4-a697-a666d09688dc" xlink:to="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_InitialDamagesMember_019d403f-62c5-7f04-8cee-d8102babe736" xlink:href="lh-20260331.xsd#lh_InitialDamagesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:to="loc_lh_InitialDamagesMember_019d403f-62c5-7f04-8cee-d8102babe736" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_EnhancedDamagesMember_019d403f-62c5-70cc-8756-e19e5f79c91f" xlink:href="lh-20260331.xsd#lh_EnhancedDamagesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:to="loc_lh_EnhancedDamagesMember_019d403f-62c5-70cc-8756-e19e5f79c91f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_RoyaltyDamagesMember_019d403f-62c5-7860-86f0-c1db15620daa" xlink:href="lh-20260331.xsd#lh_RoyaltyDamagesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:to="loc_lh_RoyaltyDamagesMember_019d403f-62c5-7860-86f0-c1db15620daa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SupplementalDamagesMember_019d403f-62c5-7d9f-b020-5f2a0c436b70" xlink:href="lh-20260331.xsd#lh_SupplementalDamagesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_LossContingencyDamagesTypeDomain_019d403f-62c5-7917-9051-77ab2247b0ab" xlink:to="loc_lh_SupplementalDamagesMember_019d403f-62c5-7d9f-b020-5f2a0c436b70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesSoughtValue_019d403f-62c5-76b3-80ba-a756b6451031" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyDamagesSoughtValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019d403f-62c5-70a8-8bfa-c86f36312a8a" xlink:to="loc_us-gaap_LossContingencyDamagesSoughtValue_019d403f-62c5-76b3-80ba-a756b6451031" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesAwardedValue_019d403f-62c5-734d-a163-b30491b4ddeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyDamagesAwardedValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019d403f-62c5-70a8-8bfa-c86f36312a8a" xlink:to="loc_us-gaap_LossContingencyDamagesAwardedValue_019d403f-62c5-734d-a163-b30491b4ddeb" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/FAIRVALUEMEASUREMENTSANDDERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#FAIRVALUEMEASUREMENTSANDDERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/FAIRVALUEMEASUREMENTSANDDERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-62c5-781c-b513-f4bbf287e7b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-62c5-781c-b513-f4bbf287e7b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-62c5-781c-b513-f4bbf287e7b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-62c5-781c-b513-f4bbf287e7b7" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-62c5-781c-b513-f4bbf287e7b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-62c5-7006-ad35-371a553996d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_019d403f-62c5-781c-b513-f4bbf287e7b7" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_019d403f-62c5-7006-ad35-371a553996d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019d403f-62c5-798e-b8a9-7a442048ed75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019d403f-62c5-798e-b8a9-7a442048ed75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-798e-b8a9-7a442048ed75_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019d403f-62c5-798e-b8a9-7a442048ed75" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-798e-b8a9-7a442048ed75_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-78a3-b405-31608f2b90c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019d403f-62c5-798e-b8a9-7a442048ed75" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-78a3-b405-31608f2b90c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019d403f-62c5-7f46-ba69-030244a4b0a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-78a3-b405-31608f2b90c5" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019d403f-62c5-7f46-ba69-030244a4b0a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019d403f-62c5-7145-9c09-4ea90375f1ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-78a3-b405-31608f2b90c5" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019d403f-62c5-7145-9c09-4ea90375f1ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019d403f-62c5-73cd-9b0f-e1da78082367" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019d403f-62c5-78a3-b405-31608f2b90c5" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019d403f-62c5-73cd-9b0f-e1da78082367" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019d403f-62c5-7010-9a94-842acf40437e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019d403f-62c5-7010-9a94-842acf40437e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-62c5-7010-9a94-842acf40437e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-62c5-7010-9a94-842acf40437e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-62c5-7010-9a94-842acf40437e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019d403f-62c5-7986-893d-290c69760bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019d403f-62c5-7010-9a94-842acf40437e" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019d403f-62c5-7986-893d-290c69760bd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SeniorNotesDue2031Member_019d403f-62c5-7537-800f-98430fc81145" xlink:href="lh-20260331.xsd#lh_SeniorNotesDue2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019d403f-62c5-7986-893d-290c69760bd4" xlink:to="loc_lh_SeniorNotesDue2031Member_019d403f-62c5-7537-800f-98430fc81145" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SwapTypeAxis_019d403f-62c5-7e42-abc9-0d3bd32d2217" xlink:href="lh-20260331.xsd#lh_SwapTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_lh_SwapTypeAxis_019d403f-62c5-7e42-abc9-0d3bd32d2217" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SwapTypeDomain_019d403f-62c5-7e42-abc9-0d3bd32d2217_default" xlink:href="lh-20260331.xsd#lh_SwapTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lh_SwapTypeAxis_019d403f-62c5-7e42-abc9-0d3bd32d2217" xlink:to="loc_lh_SwapTypeDomain_019d403f-62c5-7e42-abc9-0d3bd32d2217_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:href="lh-20260331.xsd#lh_SwapTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lh_SwapTypeAxis_019d403f-62c5-7e42-abc9-0d3bd32d2217" xlink:to="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SeniorNotesDue2031Member_019d403f-62c5-7c1c-b50f-361d94955701" xlink:href="lh-20260331.xsd#lh_SeniorNotesDue2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:to="loc_lh_SeniorNotesDue2031Member_019d403f-62c5-7c1c-b50f-361d94955701" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2029MaturingSwapAgreementsMember_019d403f-62c5-7254-923d-826758e40277" xlink:href="lh-20260331.xsd#lh_A2029MaturingSwapAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:to="loc_lh_A2029MaturingSwapAgreementsMember_019d403f-62c5-7254-923d-826758e40277" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2034MaturingSwapAgreementsMember_019d403f-62c5-72c9-8231-a9d9fec151eb" xlink:href="lh-20260331.xsd#lh_A2034MaturingSwapAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:to="loc_lh_A2034MaturingSwapAgreementsMember_019d403f-62c5-72c9-8231-a9d9fec151eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_A2031SwapAgreementsMember_019d403f-62c5-7496-9d96-5370e27d5e15" xlink:href="lh-20260331.xsd#lh_A2031SwapAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:to="loc_lh_A2031SwapAgreementsMember_019d403f-62c5-7496-9d96-5370e27d5e15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SwapAgreementsMember_019d403f-62c5-7eb0-9338-17b3ae13d3e7" xlink:href="lh-20260331.xsd#lh_SwapAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lh_SwapTypeDomain_019d403f-62c5-7071-b0e3-13cde634e67f" xlink:to="loc_lh_SwapAgreementsMember_019d403f-62c5-7eb0-9338-17b3ae13d3e7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-7594-90e6-b2394ca7e310" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-7594-90e6-b2394ca7e310" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7594-90e6-b2394ca7e310_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-7594-90e6-b2394ca7e310" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7594-90e6-b2394ca7e310_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7090-9dba-b8dc9c30ee2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-7594-90e6-b2394ca7e310" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7090-9dba-b8dc9c30ee2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019dba96-d51c-739c-9481-1794bea501ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019d403f-62c5-7bde-8e6f-fbf2ff0deff7" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019dba96-d51c-739c-9481-1794bea501ac" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-739c-9481-1794bea501ac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019dba96-d51c-739c-9481-1794bea501ac" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-739c-9481-1794bea501ac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-77bd-aa76-c0857fcab449" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019dba96-d51c-739c-9481-1794bea501ac" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-77bd-aa76-c0857fcab449" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_019dba96-f2b5-7247-a916-3480a6b2726e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-77bd-aa76-c0857fcab449" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_019dba96-f2b5-7247-a916-3480a6b2726e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_OtherLiabilitiesNoncurrentMember_019dba97-4856-7104-a23d-e52c8471397d" xlink:href="lh-20260331.xsd#lh_OtherLiabilitiesNoncurrentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019dba96-d51c-77bd-aa76-c0857fcab449" xlink:to="loc_lh_OtherLiabilitiesNoncurrentMember_019dba97-4856-7104-a23d-e52c8471397d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts_019d403f-62c5-7d09-a677-717758cf0dbc" xlink:href="lh-20260331.xsd#lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisNoncontrollingInterestPuts_019d403f-62c5-7d09-a677-717758cf0dbc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019d403f-62c5-7c5f-bb2b-d58a9ccffa99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019d403f-62c5-7c5f-bb2b-d58a9ccffa99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueHedgesAtFairValueNet_019d403f-62c5-7d1b-ac1e-3766e60aadc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueHedgesAtFairValueNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_FairValueHedgesAtFairValueNet_019d403f-62c5-7d1b-ac1e-3766e60aadc3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashSurrenderValueFairValueDisclosure_019d403f-62c5-7ad8-8eaa-770c3f684537" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashSurrenderValueFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_CashSurrenderValueFairValueDisclosure_019d403f-62c5-7ad8-8eaa-770c3f684537" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationPlanAssets_019d403f-62c5-7b3f-8ab7-33b77184815b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredCompensationPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_DeferredCompensationPlanAssets_019d403f-62c5-7b3f-8ab7-33b77184815b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability_019d403f-62c5-7622-9351-48d3b30b557c" xlink:href="lh-20260331.xsd#lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_lh_FairValueLiabilitiesMeasuredOnRecurringBasisDeferredCompensationLiability_019d403f-62c5-7622-9351-48d3b30b557c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019d403f-62c5-7746-ac68-a7baa1f9a406" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019d403f-62c5-7746-ac68-a7baa1f9a406" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_SeniorNotesFairValue_019d403f-62c5-7902-bba5-59f534795a7d" xlink:href="lh-20260331.xsd#lh_SeniorNotesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_lh_SeniorNotesFairValue_019d403f-62c5-7902-bba5-59f534795a7d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedBorrowingInterestRate_019d403f-62c5-7df3-8314-904c611886a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubordinatedBorrowingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_SubordinatedBorrowingInterestRate_019d403f-62c5-7df3-8314-904c611886a1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_019d403f-62c5-7ca9-823f-1ac3eaabd49e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_DerivativeNotionalAmount_019d403f-62c5-7ca9-823f-1ac3eaabd49e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019d403f-62c5-7274-8174-7776f6f38fa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019d403f-62c5-7274-8174-7776f6f38fa2" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_019d403f-62c5-7efd-ad71-001bf953c13e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019d403f-62c5-77bd-baec-5de117209016" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_019d403f-62c5-7efd-ad71-001bf953c13e" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/DERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESEffectsofInterestRateSwaponOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#DERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESEffectsofInterestRateSwaponOtherComprehensiveIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/DERIVATIVEINSTRUMENTSANDHEDGINGACTIVITIESEffectsofInterestRateSwaponOtherComprehensiveIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_019d403f-62c5-7c98-a9ed-62bc94184273" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_019d403f-62c5-77de-b6ee-00b921fadaa0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_019d403f-62c5-7c98-a9ed-62bc94184273" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_019d403f-62c5-77de-b6ee-00b921fadaa0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-78bc-a07e-479e4d9f4925" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_019d403f-62c5-77de-b6ee-00b921fadaa0" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-78bc-a07e-479e4d9f4925" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-78bc-a07e-479e4d9f4925_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-78bc-a07e-479e4d9f4925" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-78bc-a07e-479e4d9f4925_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7841-bfad-f0c6a8b43e74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019d403f-62c5-78bc-a07e-479e4d9f4925" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7841-bfad-f0c6a8b43e74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CurrencySwapMember_019d403f-62c5-7037-8259-3295a897125b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CurrencySwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019d403f-62c5-7841-bfad-f0c6a8b43e74" xlink:to="loc_us-gaap_CurrencySwapMember_019d403f-62c5-7037-8259-3295a897125b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_OtherComprehensiveIncomeLossCrossCurrencySwaps_019d403f-62c5-7f05-bb95-c88a98ef7541" xlink:href="lh-20260331.xsd#lh_OtherComprehensiveIncomeLossCrossCurrencySwaps"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_019d403f-62c5-7c98-a9ed-62bc94184273" xlink:to="loc_lh_OtherComprehensiveIncomeLossCrossCurrencySwaps_019d403f-62c5-7f05-bb95-c88a98ef7541" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/NONCONTROLLINGINTERESTPUTSDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#NONCONTROLLINGINTERESTPUTSDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/NONCONTROLLINGINTERESTPUTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestLineItems_019d403f-62c5-7c2f-ba31-27c8e62faf28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestTable_019d403f-62c5-760c-a2eb-92709be59108" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MinorityInterestTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MinorityInterestLineItems_019d403f-62c5-7c2f-ba31-27c8e62faf28" xlink:to="loc_us-gaap_MinorityInterestTable_019d403f-62c5-760c-a2eb-92709be59108" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019d403f-62c5-7dd9-a786-5302d2fe105b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MinorityInterestTable_019d403f-62c5-760c-a2eb-92709be59108" xlink:to="loc_dei_LegalEntityAxis_019d403f-62c5-7dd9-a786-5302d2fe105b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019d403f-62c5-7dd9-a786-5302d2fe105b_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019d403f-62c5-7dd9-a786-5302d2fe105b" xlink:to="loc_dei_EntityDomain_019d403f-62c5-7dd9-a786-5302d2fe105b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019d403f-62c5-7c58-a201-001c2df04207" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019d403f-62c5-7dd9-a786-5302d2fe105b" xlink:to="loc_dei_EntityDomain_019d403f-62c5-7c58-a201-001c2df04207" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.labcorp.com/role/BUSINESSSEGMENTINFORMATIONDetails" xlink:type="simple" xlink:href="lh-20260331.xsd#BUSINESSSEGMENTINFORMATIONDetails"/>
  <link:definitionLink xlink:role="http://www.labcorp.com/role/BUSINESSSEGMENTINFORMATIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-791a-b373-3e60216408bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-791a-b373-3e60216408bd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7fc9-8d01-a05f549b13a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-791a-b373-3e60216408bd" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7fc9-8d01-a05f549b13a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-62c5-7fc9-8d01-a05f549b13a7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7fc9-8d01-a05f549b13a7" xlink:to="loc_us-gaap_SegmentDomain_019d403f-62c5-7fc9-8d01-a05f549b13a7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019d403f-62c5-7170-8fc9-865819f7c1d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019d403f-62c5-7fc9-8d01-a05f549b13a7" xlink:to="loc_us-gaap_SegmentDomain_019d403f-62c5-7170-8fc9-865819f7c1d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_DiagnosticsMember_019d403f-62c5-71c9-bf53-872d0cada476" xlink:href="lh-20260331.xsd#lh_DiagnosticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-62c5-7170-8fc9-865819f7c1d7" xlink:to="loc_lh_DiagnosticsMember_019d403f-62c5-71c9-bf53-872d0cada476" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-62c5-7060-b68c-1fd9534d2143" xlink:href="lh-20260331.xsd#lh_BiopharmaLaboratoryServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-62c5-7170-8fc9-865819f7c1d7" xlink:to="loc_lh_BiopharmaLaboratoryServicesMember_019d403f-62c5-7060-b68c-1fd9534d2143" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateMember_019d403f-62c5-75ad-bb79-ed76463800c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019d403f-62c5-7170-8fc9-865819f7c1d7" xlink:to="loc_us-gaap_CorporateMember_019d403f-62c5-75ad-bb79-ed76463800c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019d403f-62c5-791a-b373-3e60216408bd" xlink:to="loc_srt_ConsolidationItemsAxis_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8" xlink:to="loc_srt_ConsolidationItemsDomain_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019d403f-62c5-765d-8557-514aa112548e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019d403f-62c5-7d5e-ad17-fb9b2f5e05c8" xlink:to="loc_srt_ConsolidationItemsDomain_019d403f-62c5-765d-8557-514aa112548e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019d403f-62c5-7217-a75d-5bb6c517c27c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019d403f-62c5-765d-8557-514aa112548e" xlink:to="loc_us-gaap_OperatingSegmentsMember_019d403f-62c5-7217-a75d-5bb6c517c27c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_019d403f-62c5-738c-8633-0e8e3634317d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019d403f-62c5-765d-8557-514aa112548e" xlink:to="loc_us-gaap_IntersegmentEliminationMember_019d403f-62c5-738c-8633-0e8e3634317d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_019d403f-62c5-784b-97df-2fe03fe3aa5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_Revenues_019d403f-62c5-784b-97df-2fe03fe3aa5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_019d403f-62c5-74a8-bda1-b5e3af88c466" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LaborAndRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_LaborAndRelatedExpense_019d403f-62c5-74a8-bda1-b5e3af88c466" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SuppliesExpense_019d403f-62c5-7bd6-b975-0baeebb2140f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SuppliesExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_SuppliesExpense_019d403f-62c5-7bd6-b975-0baeebb2140f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_Freight_019d403f-62c5-789f-8f18-1fab875b541c" xlink:href="lh-20260331.xsd#lh_Freight"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_lh_Freight_019d403f-62c5-789f-8f18-1fab875b541c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019d403f-62c5-7a57-b7c8-7fde12f51be6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_Depreciation_019d403f-62c5-7a57-b7c8-7fde12f51be6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCostAndExpenseOperating_019d403f-62c5-78b7-b267-80e8a3862067" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCostAndExpenseOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_OtherCostAndExpenseOperating_019d403f-62c5-78b7-b267-80e8a3862067" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019d403f-62c5-7624-a6ba-2fc559967cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_OperatingIncomeLoss_019d403f-62c5-7624-a6ba-2fc559967cd8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019d403f-62c5-7301-a381-b056c1605c5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019d403f-62c5-7301-a381-b056c1605c5f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_NetRestructuringCharges_019d403f-62c5-7182-9586-8053dd8e3e25" xlink:href="lh-20260331.xsd#lh_NetRestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_lh_NetRestructuringCharges_019d403f-62c5-7182-9586-8053dd8e3e25" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_019d403f-62c5-703f-8069-6cbd496d5903" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_GoodwillAndIntangibleAssetImpairment_019d403f-62c5-703f-8069-6cbd496d5903" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_019d403f-62c5-741f-b8d1-6ea5ccbe93ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_InterestExpense_019d403f-62c5-741f-b8d1-6ea5ccbe93ec" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_019d403f-62c5-70ac-9c98-fda4e6e7c08b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_InvestmentIncomeNet_019d403f-62c5-70ac-9c98-fda4e6e7c08b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019d403f-62c5-7205-8d88-c0fe0e5bb6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019d403f-62c5-7205-8d88-c0fe0e5bb6e5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019d403f-62c5-79df-b31f-50cff1e597fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019d403f-62c5-79df-b31f-50cff1e597fc" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_019d403f-62c5-7428-8192-c1039b7b77f4" xlink:href="lh-20260331.xsd#lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019d403f-62c5-72b5-89d1-2eb09e3b0c64" xlink:to="loc_lh_IncomeLossfromContinuingOperationsBeforeIncomeTaxesandNoncontrollingInterest_019d403f-62c5-7428-8192-c1039b7b77f4" xlink:type="arc" order="14"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_KathrynW.KyleMember_019d403f-60a3-76e6-b2ab-1c008b16e6ac" xlink:href="lh-20260331.xsd#lh_KathrynW.KyleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_lh_KathrynW.KyleMember_019d403f-60a3-76e6-b2ab-1c008b16e6ac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_BryanT.VaughnMember_019dba61-40fd-7938-a47e-1025f3b01e03" xlink:href="lh-20260331.xsd#lh_BryanT.VaughnMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_lh_BryanT.VaughnMember_019dba61-40fd-7938-a47e-1025f3b01e03" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#TradingArrangmentsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllTradingArrangementsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllTradingArrangementsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_lh_KathrynW.KyleTradingArrangementStockOptionsMember_019d403f-60a3-7b7a-b103-c1e0aea4e494" xlink:href="lh-20260331.xsd#lh_KathrynW.KyleTradingArrangementStockOptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_lh_KathrynW.KyleTradingArrangementStockOptionsMember_019d403f-60a3-7b7a-b103-c1e0aea4e494" xlink:type="arc" order="1"/>
  </link:definitionLink>
</link:linkbase>
