OBJECTIVE: To investigate the efficacy and safety of certolizumab pegol (CZP) in a broad population of patients with active RA. METHODS: In this 12-week, double-blind period of the phase IIIb trial, RA patients with inadequate response to at least one DMARD were randomized 4:1 to CZP (400 mg at weeks 0, 2 and 4, followed by 200 mg every 2 weeks) or placebo (every 2 weeks) plus current therapy stratified by previous TNF inhibitor use, concomitant methotrexate use and disease duration (<2 vs ≥2 years). The primary outcome was ACR20 response rate at week 12. RESULTS: Of 1063 patients (CZP = 851; placebo = 212), 37.6% had previous TNF inhibitor use. Baseline mean HAQ Disability Index (HAQ-DI) and DAS 28-joint assessment-ESR [DAS28(ESR)] values were 1.5 and 6.4 in the CZP group, and 1.6 and 6.4 in the placebo group, respectively. The primary endpoint was significant (week 12 ACR20, CZP vs placebo: 51.1 vs 25.9%; P < 0.001); differences were noted at week 2 (31.8 vs 8.5%; P < 0.001). HAQ-DI and DAS28(ESR) change from baseline and ACR50 were significant from week 2. Week 12 ACR20 responses were similar across CZP patient subgroups regardless of concomitant DMARD use at baseline. Adverse and serious adverse events were comparable between CZP and placebo, with no new safety signals. CONCLUSION: CZP was associated with rapid and consistent clinical responses and improved physical function in a diverse group of RA patients, irrespective of concomitant or previous therapy. TRIAL REGISTRATION: ClinicalTrials.gov, http://clinicaltrials.gov/, NCT00717236.