Diflunisal, a long acting antiinflammatory analgesic was compared in high (1000 mg daily) and low (750 mg daily) doses with placebo in a randomized, double blind study of 6 weeks' duration in patients with osteoarthritis of the knee. Two hundred twenty-seven patients from 47 centers completed the study--high dose 69, low dose 88 and placebo 70. Pain relief was significantly greater with both doses of diflunisal than with placebo. Both patient and investigator global opinions reflected significantly greater efficacy with diflunisal. Although there was a trend in favour of the higher dose, no statistically significant differences in efficacy were found between the 2 doses of diflunisal studied. Overall adverse reactions with diflunisal were no more frequent than with placebo, but gastrointestinal side effects were significantly greater with the higher dose.