166 patients with an acetabular fracture treated surgically were randomized to receive postoperative indomethacin or localized irradiation as potential prophylaxis against heterotopic ossification. There were also sixteen additional patients who were randomized but did not receive treatment with either prophylactic regime that was unable to comply with the regimen to which they were assigned. At an average follow-up of 14 months, radiographs were assessed for the extent of heterotopic ossification. The difference between groups in terms of ossification was not significant. All patients in the untreated group developed heterotopic ossification. Grade I, II, and III heterotopic ossification did not decrease hip range of motion except in flexion.