This report summarizes the results of a 17-investigator multicenter six-month randomized double-blind parallel group study. The safety and efficacy of nabumetone 1,000 mg taken at bedtime was compared with that of aspirin 900 mg four times daily in the treatment of adult patients with active class II or III classical or definite rheumatoid arthritis. Two hundred sixty-four patients were entered into the study. Two hundred fifty-seven (126 nabumetone and 131 aspirin) patients were evaluable for safety. Two hundred thirty-four (113 nabumetone and 121 aspirin) patients were evaluable for efficacy. There was significant improvement in each of six clinical measurements of efficacy in both treatment groups and little difference between groups. The somewhat greater improvement in articular index and duration of morning stiffness in the nabumetone-treated group did not reach statistical significance. There was an equal percentage of patient withdrawal for lack of efficacy in each group. Overall, the rate of patient withdrawal due to adverse experiences was greater (p = 0.01) for aspirin-treated patients. These experiences were usually dispepsia, abdominal pain, and tinnitus. It was concluded that nabumetone was an effective anti-inflammatory drug in the treatment of rheumatoid arthritis with less toxicity than aspirin.