A double-blind crossover trial in 50 patients with rheumatoid arthritis was carried out to compare the clinical effectiveness of 800 mg. azapropazone per day with 100 mg. indomethacin per day. Blood plasma concentrations of azapropazone were also studied in 10 patients. Patients received both drugs for a 3-week period, the order of treatment being determined at random. Although there were no marked differences in any of the objective measurements, patients assessments of response to and preference for each drug treatment period suggested that azapropazone produced significant improvement compared to that with indomethacin. Plasma levels of azapropazone remained remarkably constant for each patient. Side-effects with both drugs were mild and transient, and there were no marked variations from normal in any of the laboratory parameters measured.