An open, multicentre trial was carried out in 140 patients with coxarthroxis and/or gonarthrosis to assess the effectiveness of treatment with diftalone. The trial lasted 4 weeks with a drug dosage decreasing from 1000 mg (Week 1) to 750 mg (Week 2), and to 500 mg (Weeks 3 and 4). In almost all of the objective and subjective assessments used for the measurement of effectiveness there was a statistically significant improvement with treatment, which was dose related, the greatest improvement being seen during the first week. Tolerance of treatment was assessed as good in the majority of patients and again, side-effects appeared to be dose related.