A double-blind trial was carried out in 24 patients with osteoarthritis of the knee or hip to compare the efficacy and tolerance of oxaprozin with that of naproxen. Patients were assigned at random to receive fixed doses of either 1200 mg oxaprozin once daily or 250 mg naproxen 3-times daily over a period of 8 weeks. Assessments made on entry and after 4 and 8 weeks of treatment showed that in the oxaprozin group there were significant mean decreases, indicating improvement in patient's condition, with respect to observer's opinion, patient's opinion, pain intensity and activity impairment at both on-therapy visits. In the naproxen group, there were significant mean decreases with respect to observer's opinion, patient's opinion, pain intensity and time to walk 15 metres. None of the mean differences between the groups was statistically significant. Adverse effects were reported for 3 of the 12 oxaprozin patients and 6 of the 12 naproxen patients. The specific adverse effects noted for more than 1 patient were diarrhoea for oxaprozin and dyspepsia for naproxen. No difference between the groups was statistically significant from this point of view. Laboratory determinations showed no toxicity in either group. It is concluded that once-daily oxaprozin is an effective and well-tolerated form of treatment for osteoarthritis, equivalent to naproxen given 3-times daily.