Disease-modifying antirheumatic drug (DMARD) combination therapies are used widely, but there have been few reports clearly demonstrating that combination therapy is more effective than DMARD monotherapy. We conducted a multicenter, double-blind controlled trial in order to clarify that the combination of methotrexate and bucillamine is more effective than either alone. The subjects of this study were 71 patients with active rheumatoid arthritis within 2 years of onset. Dosages were 8 mg methotrexate with 5 mg folic acid per week (MTX group), 200 mg bucillamine per day (BUC group), or both MTX and BUC (combination group). Clinical effects and adverse reactions were observed for 96 weeks. The ACR 20 response rate was 79.2% in the combination group, significantly higher than the rates of 43.5% for the MTX group (P = 0.008) and 45.8% for the BUC group (P = 0.0178). The cumulative survival curve of maintaining the ACR 20 response was significantly higher in the combination group than in the MTX and BUC groups (P = 0.0123 and P = 0.0088, respectively). The mean increase in the total Sharp score over 96 weeks was 12.6 +/- 9.0 in the combination group, significantly lower (P = 0.0468) than the value of 28.0 +/- 28.3 for the single DMARD (combined MTX and BUC) group. The incidence of adverse reactions did not differ significantly between the three groups. It was concluded that the combination therapy with MTX and BUC showed significantly higher clinical efficacy than either of the single DMARD therapies.