Altogether 1,912 patients have been enrolled in studies of nabumetone in the United States. They were all patients with osteoarthritis or rheumatoid arthritis. After completing participation within randomised studies patients were entered into an open study which continued for up to five years. These cases provide longterm efficacy and safety data for the use of nabumetone. About 25% of patients were withdrawn because of lack of efficacy; most of these withdrawals were in the first year of therapy. There was a similar picture with side-effects. About 12% of patients were withdrawn, mostly in the first year. Many patients continued to take nabumetone for five years without adverse effects and with continuing efficacy. The numbers of patients in whom nabumetone was stopped due to lack of efficacy or an adverse reaction was similar in those aged over 65 years and in patients aged less than 65 years. These results show that nabumetone is a safe and effective anti-inflammatory drug in long term clinical use.