An open study was carried out in 46 patients with osteoarthritis of the hip to compare the efficacy and tolerance of treatment with ketoprofen given either as 100 mg capsules twice daily or as 2 capsules of 100 mg ketoprofen in a controlled-release formulation given once daily. The results of subjective and objective assessments before and during 3-months' treatment in the 48 patients who completed the trial showed both treatments produced improvement in all parameters, except for the time taken for inactivity stiffness to develop, and there was no significant difference between treatments in terms of efficacy. The controlled-release preparation, however, was significantly better tolerated than the ordinary capsule form. Minor haematological and biochemical changes during treatment were noted but these were not of clinical importance. Six patients, 2 receiving the controlled-release and 4 receiving the ordinary formulation of ketoprofen, were withdrawn because of lack of efficacy or unacceptable side-effects.