Forty-seven patients with active rheumatoid arthritis were entered into a single-blind study to compare the safety and efficacy of nabumetone with naproxen. Patients were randomly assigned to receive a daily dose of 2 g of nabumetone or 1 g of naproxen after a placebo-washout period of at least three days. In addition to the usual clinical and laboratory measurements of disease activity, thermographic assessment was carried out at each visit and the heat distribution index was calculated. In addition, the safety and tolerance were assessed. Patients had improvement in severity of pain, severity and duration of morning stiffness, and articular index, and there was no statistical difference between the two treatment groups. By the end of six months, 13 patients had withdrawn from treatment; five patients in the naproxen treatment group and five in the nabumetone group were withdrawn from the study due to lack of efficacy. In no patient from the nabumetone group did an adverse reaction develop. In one patient in the naproxen group, a severe decrease in white blood cell count developed, and a skin rash and swollen gums developed in another patient. We conclude that nabumetone is well tolerated and of equal efficacy to naproxen in the treatment of active rheumatoid arthritis.