In 2 multicenter, double blind studies, the efficacy and safety of diclofenac, 150 mg/day, were compared with those of aspirin, 3.6 g/day, in 194 patients with rheumatoid arthritis (RA) in Study 1 and with those of naproxen, 1000 mg/day, in 223 patients with RA in Study 2. After single blind, placebo washout periods of 2 days to 2 weeks, patients entered 12-week treatment periods in each study. In both studies, diclofenac, aspirin, and naproxen produced statistically significant improvement (p less than or equal to 0.01) from baseline in all primary efficacy variables at each assessment visit. There were no significant differences between treatments. In both studies, significantly fewer (p less than or equal to 0.05) patients receiving diclofenac experienced adverse effects compared to the aspirin and naproxen groups. Significantly fewer (p less than 0.05) patients in the diclofenac group compared to the aspirin group discontinued the trial due to side effects (primarily tinnitus and deafness). In Study 2, fewer patients in the diclofenac group discontinued the trial due to adverse effects than in the naproxen group. In conclusion, diclofenac, aspirin, and naproxen demonstrated similar efficacy; however, diclofenac was significantly better tolerated than either aspirin or naproxen.