Purpose/Objective: We report our preliminary results of a randomized trial comparing one single dose of 400 cGy versus 700 cGy given postoperatively in an attempt to prevent heterotopic ossification after total hip arthroplasty.Materials & Methods: From 09/1993 and 05/1996, over 800 total hip replacements were performed at our hospital. From this group of patients, 120 hips in 114 high-risk patients (14%) were enrolled in a randomized trial to determine if 400 cGy (Group A) is as efficacious as 700 cGy (Group B) in preventing heterotopic ossification. In Group A, there were 42 males (46 hips) and 12 females (12 hips) with a mean age of 60 (range 41-79); with 18 primary cementless femoral components (33%), 30 primary cemented stems (55%) and 10 revisions. In Group B, there were 30 males (32 hips) and 30 females (31 hips) with a median age of 59 (range 41-85); with 12 primary cementless femoral components (20%), 44 primary cemented stems (73%) and 6 revisions. All acetabular components were of the cementless type. Patients were randomized to receive either 400 cGy or 700 cGy in one fraction. Radiotherapy is given within 48 hours post-operatively using paired anterior and posterior fields, with blocking of the cementless acetabular component and the femoral component.Results: All 114 patients were available for a minimum follow-up of 6 months (range 6-30 months). None of the arthroplasties has failed at the latest follow-up. There were no radiation therapy complications noted. Statistical analysis revealed no difference in the distribution of patients in either group according to age, sex, primary or revision arthroplasty, cemented or cementless femoral component fixation, preoperative heterotopic ossification risk, or surgical approach. Of the 58 hips in Group A, heterotopic ossification was graded as Grade 0 in 24 hips, Grade I in 10 hips, Grade II in 18 hips, Grade III in 6 hips, with no cases of Grade IV. Of the 63 hips in Group B, heterotopic ossification was graded as Grade 0 in 34 hips, Grade I in 12 hips, Grade II in 10 hips, Grade III in 7 hips, with no cases of Grade IV. Progression to a higher grade of heterotopic ossification was observed in 14 patients (26%) in Group A and 14 patients (23%) in Group B, with no statistically significant difference between the two groups.Conclusion: The results of our study support the hypothesis that a single fraction of 400 cGy is as efficacious as 700 cGy in the prevention of heterotopic ossification after total hip arthroplasty.