Forty-four fractures of the acetabulum that had been treated with open reduction and internal fixation necessitating dissection of the gluteal muscles were reviewed radiographically for the development of heterotopic bone. Grade-2 (Brooker classification) or more severe heterotopic ossification was seen in thirteen (50 per cent) of twenty-six patients who did not receive indomethacin but in only one (5.5 per cent) of eighteen patients who received indomethacin for six weeks postoperatively. In the patients who did not receive indomethacin, the maximum amount and extent of the heterotopic bone was evident in twelve weeks. In the patients who did receive indomethacin, the heterotopic ossification did not progress after the administration of indomethacin was discontinued. We concluded that, in patients who have a fracture of the acetabulum, indomethacin provides effective prophylaxis for heterotopic bone after operative reduction with gluteal dissection.