In order to prevent heterotopic ossification (HO) after total hip replacement, 21 high risk hips were irradiated pre- and postoperatively to prevent heterotopic bone formation in the St. Paul Department of Radiation Oncology from 1993 to September, 1995. Eighteen hips in 15 patients were eligible for analysis with a minimum follow-up of 3 months. All the hips in our series were at high risk of heterotopic bone formation. All assessable patients had radiographic findings of ipsilateral or contralateral previous ectopic bone formation, 8 following total hip arthroplasty, 4 following trauma and 3 had heterotopic osteoarthritis. Four hips were treated 46 hours preoperatively, while all the others were treated on the first postoperative day. The irradiation field was limited to the hip region and radiation exposure to a rectangular field, ranging in size from 8 x 12 to 10 x 15 cm, to include the lateral aspect of the greater trochanter. We used a cobalt-60 unit. Opposed anterior-posterior fields were irradiated at midplan, with a source-to-axis distance of 80 cm; the dose was 6-8 Gy in one fraction. Preoperative irradiation appeared as effective as postoperative treatment, and more comfortable for the patients. There were neither side-effects nor failure of ossification. The treatment appeared effective in the prophylaxis of HO in 88% of patients, with clinical advantages in 67% of them.