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OBJECTIVES Describe rate of post-operative heterotopic ossification (HO) after acetabular surgery for patients that received external beam radiation (XRT) as HO prophylaxis. DESIGN Retrospective. SETTING Level I trauma center.Patients/participants: Consecutive series of patients that presented to a single level I academic trauma center over a 10-year period (2008-2018) for surgical fixation of an acetabular fracture. Patients eligible for inclusion were those that underwent surgical fixation of an acetabular fracture via a posterior (Kocher-Langenbeck), combined anterior and posterior, or extensile exposure. Patients were excluded if an isolated anterior approach was performed, or if an acute total hip arthroplasty was performed at the time of index surgery. INTERVENTION XRT. MAIN OUTCOME Severe HO (Brooker class III or IV). RESULTS The severe HO (Brooker class III or IV) rate for entire cohort was 12% (44 / 361 patients). Of these 44 patients, 30 patients were classified as Brooker III and 14 patients were classified as Brooker IV. The Brooker IV rate for the entire cohort was 4% (14/361 patients). Severe HO rates showed a declining trend over the time period examined, with a risk reduction of -1.0% per year (95% CI -2.1 to 0.2%; p=0.10). CONCLUSION To our knowledge, this is the largest single consecutive series on acetabular fracture patients that received XRT as HO prophylaxis. The overall severe HO rate was 12%, which is similar to other comparably large series data on patients that did not receive XRT after surgical fixation acetabular fractures. Although these data suggest XRT may not be beneficial when used universally for all patients, comparative studies are required to rule out the benefits of XRT for preventing HO in this population. LEVEL OF EVIDENCE Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.