OBJECTIVE: Certolizumab pegol (CZP) is known to be effective as monotherapy at a dosage of 400  mg every 4 weeks in patients with active RA who have failed DMARDs. The aim of this study was to investigate every 4-week CZP in addition to continued MTX therapy in patients with an inadequate response to MTX alone. METHODS: Patients with active RA with inadequate response to MTX, on background MTX, were randomized to double-blind treatment with CZP 400  mg or placebo every 4 weeks for 24 weeks (NCT00544154). The primary efficacy end-point was the ACR 20% improvement criteria (ACR20) response rate at Week 24. Other end-points included ACR50 and ACR70 response rates, ACR core components, 28-joint DAS (ESR) with three variables (DAS28-3) and health-related quality-of-life outcomes in addition to safety. RESULTS: Of 247 randomized patients, 126 received CZP and 121 received placebo, in addition to MTX. ACR20 response rates were 45.9 vs 22.9%, respectively [P < 0.001 analysed by the Cochran-Mantel-Haenszel (CMH) method], with improvements being apparent from Week 1. Statistically significant improvements over placebo were seen with CZP for ACR50, ACR core components, DAS28-3 and physical functioning. Rates of treatment-related adverse events were similar between groups (25.0 vs 27.7%), and there were no deaths or serious opportunistic infections. CONCLUSION: CZP 400  mg every 4 weeks plus MTX demonstrated a favourable risk-benefit profile with rapid onset of action in RA patients with an inadequate response to an earlier MTX therapy.