Data from more than 100,000 patients in foreign and United States trials provide substantial evidence of diclofenac's safety and tolerability. Adverse experiences were infrequent and generally mild or transient. In United States short-term trials, the frequency and severity of side effects compared favorably with rates for placebo, aspirin, and other NSAIDs. The drop-out rate for therapeutic reasons (adverse effects or lack of efficacy) was lower for diclofenac than for any of the comparative treatments. In long-term trials, diclofenac has been taken safely for a year or more. The incidence of adverse experiences reported for older patients (greater than or equal to 65 years) treated with diclofenac did not generally differ from that reported for younger patients. No significant differences in the incidence of hepatic problems were detected between diclofenac and other active treatments in U.S. trials. Finally, foreign post-marketing data on adverse experiences show that diclofenac is one of the safest agents of its kind for the treatment of a broad range of rheumatic conditions.