Eighty-six hips in eighty-five patients who were considered to be at risk for heterotopic ossification following a total hip arthroplasty were prospectively randomized or assigned to one of two treatment groups that received a single 800-centigray dose of limited-field radiation either preoperatively (Group I) or postoperatively (Group II). The risk factors for postoperative heterotopic ossification included previous heterotopic ossification following an operation about the hip, hypertrophic osteoarthrosis or post-traumatic osteoarthrosis characterized by the presence of extensive osteophytes, radiographic evidence of diffuse idiopathic skeletal hyperostosis, and ankylosing spondylitis. The hips in Group I were irradiated within 6.1 hours before the operation and those in Group II, within 51.3 hours after the operation. Either extra-field ossification or heterotopic ossification was observed in forty-one (48 per cent) of the eighty-six hips, thereby confirming the high risk for the population in this study. After a minimum duration of follow-up of six months, thirty-seven (76 per cent) of the forty-nine hips that had been treated with preoperative irradiation exhibited no new heterotopic ossification and eleven, progression to grade-I or II ossification. The remaining hip in that group was in a woman who had Paget disease as well as previous grade-IV (ankylosing) heterotopic ossification about the ipsilateral hip; heterotopic ossification progressed from grade II on the radiographs made immediately after the index revision procedure to grade III at the most recent follow-up assessment. Of the thirty-seven hips that had been treated with postoperative irradiation, twenty-seven (73 per cent) exhibited no new heterotopic ossification and nine had progression from grade-0 to grade-I ossification. The remaining hip in that group was in a man who had Parkinson disease and previous grade-III ossification about the ipsilateral hip; heterotopic ossification progressed from grade III immediately post-operatively to grade IV at the time of the most recent evaluation. Extra-field ossification was identified in twelve (24 per cent) of the forty-nine hips that had been irradiated preoperatively compared with three (8 per cent) of the thirty-seven hips that had been irradiated postoperatively (p = 0.05). Extra-field ossification was not associated with clinical symptoms of bursitis of the greater trochanter in any hip. Three of the ten hips that had a revision operation subsequently had a non-union of the greater trochanter; all three had been treated with preoperative irradiation. The findings of the present study suggest that pre-operative irradiation is effective for the prevention of heterotopic ossification following total hip arthroplasty and that it eliminates the discomfort and morbidity that are associated with conventional postoperative treatment. Furthermore, the efficacy of preoperative irradiation suggests that osteogenic precursor cells that are active in this process are derived from the local tissues within the operative field rather than from distant blood-borne cell lines.