One hundred four patients with displaced acetabular fractures surgically treated through a posterior or combined anterior-posterior approach were randomized to receive postoperative external beam radiation therapy (XRT; n=54) or no XRT (n=50). The primary outcome of interest was the incidence of severe heterotopic ossification (HO), defined as Brooker class III–IV. Secondary outcomes included any HO (Brooker classes I–IV). Radiographic outcomes were assessed at a minimum of 6 weeks postoperatively. Overall, the results of the study revealed that severe HO occurred in 6% of patients who received XRT compared to 18% in the control group. Any HO was also less frequent in the XRT group (19% vs. 34%), though this did not reach statistical significance. The findings suggest that XRT significantly reduces the incidence of severe HO and may serve as an effective prophylactic strategy.