Thirty-seven male patients with acetabular fractures requiring an extensile surgical approach were treated using a uniform protocol that was begun in July 1984. In 1987, prophylactic indomethacin was added to the protocol to study its effects on the prevention of heterotopic ossification. Nineteen patients with at least 12 months' follow-up were operated on prior to, and 16 after the initiation of the indomethacin treatment program. Two patients were lost to follow-up. The severity of heterotopic ossification was evaluated for each of these two patient groups using the Brooker classification and was correlated with hip joint mobility. Statistical analysis revealed that indomethacin was effective in decreasing the overall incidence of heterotopic ossification (p<0.01). The occurrence of severe heterotopic ossification (Brooker class III and class IV) was also significantly reduced (p<0.01) and functional results thereby improved. The maximal extent of heterotopic ossification was evident by 6 weeks postoperatively in all patients. In the patients receiving indomethacin, heterotopic ossification did not progress after the drug was discontinued.