OBJECTIVE: To evaluate the safety and efficacy of abatacept plus methotrexate (MTX) over 5 years in patients with rheumatoid arthritis. METHODS: Patients were randomized to abatacept 10 or 2 mg/kg or placebo, plus MTX. Patients completing the 1-year, double-blind period entered the longterm extension, where all patients received a fixed dose of abatacept ~10 mg/kg. We describe safety analyses for all patients who received at least 1 dose of abatacept and efficacy analyses for the original ~10 mg/kg abatacept-treated group, over 5 years. RESULTS: Of the 235 abatacept- or placebo-treated patients completing the double-blind period, 219 entered the longterm extension; 130 (59.4%) were continuing at Year 5. No unexpected safety events were observed during the longterm extension compared with the double-blind period. Incidence rates of adverse events (AE) and serious AE were 489.7 and 20.0/100 patient-years in Year 1 versus 374.9 and 18.9/100 patient-years in the cumulative period, respectively. Using exploratory analyses, improvements observed at Year 1 in the 10 mg/kg group were maintained at Year 5, as assessed by ACR responses (ACR20=77.1% vs 82.7%; ACR50=53.0% vs 65.4%; ACR70=28.9% vs 40.4% at Years 1 and 5, respectively) and disease activity (Low Disease Activity State=48.2% vs 58.5%; Disease Activity Score-28-defined remission=25.3% vs 45.3% at Years 1 and 5, respectively). CONCLUSION: Abatacept maintained the efficacy observed at Year 1 over 5 years of treatment, and demonstrated consistent safety and tolerability. These data, along with relatively high retention rates, support the longterm clinical benefit provided by selective T cell costimulation modulation. Clinical trial registry: ClinicalTrials.gov; clinical trial registration number: NCT00254293.