One hundred and seventeen patients with osteoarthritis of the hip and/or knee were entered into this double-blind cross-over study of mefenamic acid and ibuprofen. Following a one week washout period, patients received either 1500 mg mefenamic acid or 1800 mg ibuprofen daily for four weeks. After a second, one-week washout period the patients were crossed over to the other active treatment for a further four weeks. Mefenamic acid was statistically significantly more effective in relieving pain at night of the affected joint. Of the 77 per cent of patients who expressed a preference for one or other of the medications, 64 per cent preferred mefenamic acid. Fifteen patients reported 19 adverse events while taking ibuprofen compared with only nine patients who reported 10 events while taking mefenamic acid.