A double-blind clinical trial was carried out on 2 parallel groups each of 20 in-and out-patients with classical or definite rheumatoid arthritis, to compare the efficacy and tolerance of proglumetacin, a new non-steroidal anti-inflammatory drug, with naproxen. Each patient received daily either 300 mg proglumetacin or 500 mg naproxen in two divided doses, at meals, over a period of 3 months. The whilst both drugs were effective in the long-term treatment of chronic rheumatoid arthritis, proglumetacin appeared to be somewhat more effective than naproxen in reducing the duration of morning stiffness, the articular inflammation score index, erythrocyte sedimentation rate and the dosage of concomitant basic medication. Few side-effects were reported but 1 of the 2 patients in the naproxen group who developed allergic reactions had to be withdrawn during the first few days of treatment.