A 12-week, double-blind, multi-centre trial was carried out in 402 out-patients with osteoarthritis to compare the efficacy and tolerability of the osmotic-release formulation of indomethacin ('Osmosin') with that of indomethacin capsules. Data from a sub-set of 178 patients aged 60 years or over were analyzed separately. The results indicated that 'Osmosin', releasing the equivalent of 7 mg indomethacin per hour over approximately 10 hours, provided efficacy comparable to or better than that of indomethacin capsules (25 mg 3-times daily) and had a better tolerance profile. Fewer patients treated with 'Osmosin' developed adverse experiences (p less than 0.05), especially central nervous system symptoms (p less than 0.05), and fewer were withdrawn from treatment because of adverse experiences (p less than 0.05). A reduction in the number of patients having gastro-intestinal symptoms was also identified. The majority of the patients maintained on one daily dose of 'Osmosin' had a good or excellent therapeutic response.