A multi-centre double-blind crossover study was carried out in 70 hospitalised patients with rheumatoid arthritis to compare the activity of diftalone, a new anti-inflammatory agen, with that of indomethacin. Patients received either 750 mg. diftalone or 100 mg. indomethacin daily for 2 weeks and were then crossed over to the alternative treatment for a further 2 weeks. Routine clinical and laboratory assessments of disease activity were carried out before the start of the trial and after each treatment period. The results indicate that both drugs produced a favourable response and that there was no statistically significant difference between them in the parameters measure, except for the erythrocyte sedimentation rate which fell only after the administration of diftalone. Patient and physician assessments on the course of the disease were in general agreement and were favourable to the same extent for both treatments. Both products were well tolerated and there were few reports of side-effects.