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OBJECTIVE: A placebo controlled, double-blind trial (DBT) was conducted for Japanese patients with active rheumatoid arthritis (RA) despite treatment with low dose methotrexate (MTX) to evaluate the efficacy and safety of infliximab. Extended treatment with infliximab was conducted in an open-label trial (OLT). METHODS: In the DBT, 147 patients were randomly assigned and treated with a placebo or 3 mg/kg or 10 mg/kg infliximab at Weeks 0, 2 and 6, combined with MTX. In the OLT, 129 patients from the DBT received 3 mg/kg infliximab every 8 weeks. RESULTS: The mean dose of MTX was 7.2 +/- 2.0 mg/week. Significantly more patients receiving 3 mg/kg (61.2%) and 10 mg/kg (52.9%)infliximab achieved a 20% improvement according to the American College of Rheumatology (ACR) criteria at Week 14, compared to placebo (23.4%) (p < 0.001). There was no significant difference in incidence of adverse events among the treatment groups. In patients receiving infliximab in the DBT, 11.6% of patients with serum infliximab just before the OLT developed antibodies to infliximab (ATI) in the OLT, whereas 62.2% of patients without serum infliximab did. In patients receiving placebo in the DBT, 43.9% developed ATI. CONCLUSION: The efficacy and safety of infliximab combined with low dose MTX were similar to those of the ATTRACT study. The data from the DBT and OLT also supported the importance of an induction treatment of infliximab, followed by a maintenance treatment without a long interval, giving stable serum concentrations in order to prevent formation of ATI.