A double-blind cross-over study of oral ketoprofen capsules 100 mg/day and phenylbutazone capsules 400 mg/day has been carried out in 44 patients with osteoarthrosis of the hip. All patients received each drug for 4 weeks. Eight patients were withdrawn for various reasons during the trial period, leaving 36 assessable patients (20 males and 16 females), whose ages ranged from 31-88 years (mean 66 years). In this series of patients the results suggest that there is little to choose in efficacy between the two drugs at these dosages. All patients showed improvement after 4 weeks' treatment on either drug and of 12 parameters measured only two showed a statistically significant favour to phenylbutazone (intermalleolar straddle and intercondylar distance; both when pain first appeared). Side-effects were reported equally with both medications, even though of necessity the protocol called for the exclusion of patients with previous intolerance or sensitivity to phenylbutazone. The study was instituted during the early evaluation of ketoprofen when the dosage of 100 mg/day was being tried; it is felt that a more significant result may possibly have emerged if the dosage had been the now recommended one of 150 mg/day.