Sixty-two hips in fifty-five patients who were considered to be at risk for postoperative heterotopic ossification were randomly divided into two groups: one received a single 800-centigray dose of limited-field radiation and the other, 1000 centigray of limited-field radiation in divided doses. The risk for heterotopic-bone formation was identified on the basis of previously described criteria, which included previous heterotopic ossification after an operation about the hip, hypertrophic osteoarthritis or post-traumatic osteoarthrosis characterized by formation of extensive osteophytes, radiographic evidence of diffuse idiopathic skeletal hyperostosis, ankylosing spondylitis, and male sex. The treatment portals excluded prosthetic surfaces that were intended for biological fixation by ingrowth of bone. At a minimum six-month follow-up, progression of heterotopic ossification had occurred in seven (21 per cent) of thirty-four hips in the first group and in six (21 per cent) of twenty-eight hips in the second group. The ossification had advanced more than one grade in only one hip. Extra-field ossification occurred in fifteen (43 per cent) of thirty-five hips that had not had previous heterotopic ossification. Since the time of the study, the treatment portal has been modified to include the lateral aspect of the greater trochanter, so that the risk of bursitis associated with ossification in this area is minimized. Single-dose limited-field radiation is effective for the prevention of heterotopic ossification, without compromise of early fixation of an uncemented implant.