Fifty patients with periarthritis of the shoulder were entered into this randomized double-blind trial, twenty-five on ibuprofen 1600 mg daily and twenty-five on diclofenac 100 mg daily, in order to compare the efficacy and side-effects of these two drugs over a 14-day treatment period. Based on forty-six patients with adequate follow-up data, statistically significant improvements within both treatments (p less than 0.001) were noted during the course of the study with respect to all of the variables studied: degree of pain, amplitude of rotation, amplitude of abduction and patient treatment efficacy as assessed by both the patient and the clinician. Approximately one-half of the patients on each treatment reported either 'good' or 'very good' treatment results at Day 14. No significant difference between the treatments was noted with respect to the amount of improvement shown for any of the variables studied. Side-effects, the majority of which were gastro-intestinal in nature, were reported for five patients on ibuprofen and six patients on diclofenac. They were severe enough to cause treatment to be stopped in six patients, three on each treatment. This trial demonstrates that ibuprofen and diclofenac are of virtually equal efficacy and tolerability in the treatment of patients with periarthritis of the shoulder.