Abstract: Purpose/Objective: The purpose was to determine if external beam radiation can prevent heterotopic ossification (HO) in patients with a history of heterotopic bone formation, either in that same hip after HO resection or the contralateral hip. We have used radiation with various dose-fractionation schemes since 1982. We reported our experience in this subset of high-risk patients. Materials/Methods: We reviewed the records and radiographs of 124 patients with a history of HO who underwent hip surgery between August 31, 1982 and July 24, 2001 and postoperative external beam radiation to prevent HO in that hip. Patients were excluded if they did not have both preoperative and follow-up Brooker Grades and follow-up greater than 6 months. Of a possible 124 patients, 84 patients met our inclusion criteria: 65 had HO in the same hip and 19 had HO in the contralateral hip. The median age was 64 years (range: 22-83). The radiation doses among the 84 patients were 20 Gy in 10 fractions, 20 Gy in 5, 10 Gy in 5, 8 Gy in 1, 7 Gy in 1, 6 Gy in 3, 6 Gy in 2, and 6 Gy in 1. The median follow-up was 20.5 months (range: 6.2-147). The preoperative and follow-up Brooker Grades were compared to determine if radiation was effective in preventing HO. Fisher's exact test was used to compare the development of HO between various groups. Results: The 65 patients with a history of HO on the same side had preoperative Brooker Grades ranging from 0 to 4, with 55 hips having preoperative Brooker Grades of 3 or 4. All 19 patients with HO on the contralateral side had a preoperative Brooker Grade of 0. In the 65 patients with HO in the same side, 14 developed no HO, 21 developed Brooker Grade 1 HO, 22 developed Grade 2 HO, four developed Grade 3 HO, and four developed Grade 4 HO. As such, 8 of 65 (12.3%) developed significant HO (defined as Brooker Grade 3 or 4). In the 19 patients with HO in the contralateral side, 10 developed no HO, three developed Brooker Grade 1 HO, four developed Grade 2 HO, and two developed Grade 3 HO. Thus, only 2 of 19 (10.5%) developed significant HO. There was no difference in the development of HO in the operated hip if the same hip or the contralateral hip was initially affected with HO (p=NS). Five patients received 6 Gy in 3 fractions after excision of their heterotopic bone in the same hip (one with preoperative Brooker Grade 2, two with Grade 3, and two with Grade 4 HO). Two developed Brooker Grade 2 and three developed Grade 4 HO in follow-up. Three of 5 patients (60%) developed significant HO. Thus, 6 Gy in 3 fractions was not effective in preventing HO when compared to the other dose-fractionation schemes as group (p=0.01). By contrast, 29 patients received 7 Gy in 1 fraction after excision of their heterotopic bone in the same hip. Three developed Brooker Grade 3 and one developed Grade 4 HO on follow-up. Thus, 4 of 29 (13.8%) treated with 700 Gy in 1 fraction developed significant HO. When compared to the other dose-fractionation schemes as a group, there was no significant difference in heterotopic bone formation (p=NS). Conclusions: External beam radiation is effective in preventing HO in bone-forming patients. Six Gy in 3 fractions is inadequate in preventing HO in these high-risk patients. For patients undergoing postoperative radiation, we recommend using 7 Gy in 1 fraction which is routinely used at our institution to prevent HO after hip surgery.