BACKGROUND: Therapeutic drug monitoring may optimize therapy for Crohn's disease (CD). AIM: To use a population pharmacokinetic model that accounts for the time-varying nature of covariates to simulate certolizumab pegol (CZP) concentrations to evaluate the exposure-response relationship for CZP in Crohn's disease. METHODS: Adults (N = 2157) with Crohn's disease were treated with CZP in nine clinical trials. Simulated CZP concentrations were compared to outcomes at weeks 6 and 26, including Crohn's disease activity index (CDAI) response (decrease from baseline >/= 100 points), remission (CDAI </= 150), C-reactive protein (CRP) </= 5 mg/L, faecal calprotectin (FC) </= 250 mug/g, and a composite endpoint of CDAI </= 150 and FC </= 250 mug/g. Multivariable analyses identified covariates associated with outcomes and receiver operating characteristic analyses determined optimal CZP concentrations. RESULTS: CZP concentrations at weeks 2, 4 and 6 were higher in patients with clinical response, remission, CRP </= 5 mg/L or FC </= 250 mug/g at week 6 than without. In multivariable analyses, higher CZP concentrations at week 6 were associated with the composite outcome at weeks 6 and 26 (P < .001). Although the exposure-response relationship varied among patients, approximate CZP concentrations of at least 36.1 mug/mL (positive predictive value [PPV] 22.8% and negative predictive value [NPV] 92.7%) and at least 14.8 mug/mL (PPV 28.0% and NPV 90.4%) at weeks 6 and 12 were associated with weeks 6 and 26 outcomes. CONCLUSIONS: An exposure-response relationship was apparent for CZP in Crohn's disease and achieving higher CZP concentrations may increase the likelihood of attaining efficacy outcomes, but this remains to be evaluated prospectively.