Data from four double-blind studies of the treatment of patients with rheumatoid arthritis or osteoarthritis were combined. For 4 to 12 weeks, 747 patients received Arthrotec, a combination of 50 mg of diclofenac and 200 micrograms of misoprostol, and 754 patients received 50 mg of diclofenac; the drugs were given twice or three times daily. The five most commonly reported adverse events were abdominal pain by 23.2% of the diclofenac/misoprostol patients and 19.8% of the diclofenac patients; diarrhea by 19.9% and 11.3%; nausea by 11.8% and 6.5%; dyspepsia by 11.2% and 7.8%; and flatulence by 8.0% and 3.1%. Other adverse events, reported by similar proportions of both treatment groups, included headache, gastritis, dizziness, vomiting, and constipation. In the diclofenac/misoprostol-treated patients, the abdominal pain and diarrhea were rated mild in 30.6% and 24.3%, moderate in 49.1% and 51.4%, and severe in 20.2% and 24.3%. Serious adverse events occurred in eight of the diclofenac/misoprostol-treated patients and in 13 of the diclofenac-treated patients; 12.6% and 10.1%, respectively, were withdrawn from the study because of adverse events. Results of laboratory tests of hepatic and renal function were similar in the two treatment groups.