Heterotopic ossification (HO) with subsequent pain and limitation of motion of the lower extremity is a common and significant problem for patients who suffer traumatic acetabular fracture (TAF). The incidence of heterotopic ossification is markedly increased for patients requiring surgical repair depending on the degree of trauma and the type of surgical repair necessary. Radiation therapy (RT) has proven to be the most effective surgical adjunct for the prevention of heterotopic ossification in patients undergoing total hip replacement (THR), but has not been reported in patients with traumatic fracture and repair. This report details an experience with patients treated at a Shock Trauma Center with extensile repair and immediate (within 48 hr) post-operative radiation therapy given as 5 daily fractions of 2 Gy in 5 to 7 days to a total dose of 10 Gy using megavoltage radiation therapy. A total of 30 consecutive patients (RT group) have been treated at our institution since June 1985. The last 20 patients treated with surgery only (non-RT group) prior to initiation of this study were used as a control group. Heterotopic ossification was seen to some degree in 50% of all radiation therapy patients, but was severe in only three of 30 (10%) of cases [three (10%) had Brooker III HO and no patients had ankylosis (Brooker IV HO)]. In contrast, some degree of heterotopic ossification was seen in 90% of the non-radiation therapy patients, and was severe in 10 of 20 (50%) of patients [seven (35%) had Brooker III HO whereas three (15%) had ankylosis (Brooker IV)]. This difference is significant for both total incidence and incidence of severe cases (p less than 0.01). This reduction in heterotopic ossification incidence approaches the magnitude reported for high-risk patients with total hip replacement. Even though the incidence of severe heterotopic ossification after radiation therapy for total hip replacement is approximately 5% and for traumatic acetabular fracture patients it is double (10%), the actual incidence of heterotopic ossification without radiation therapy is different in the two conditions. For total hip replacement, the incidence is about 30% and for traumatic acetabular fracture it is 50%. Radiation therapy has again proven itself to be an excellent surgical adjunct to prevent heterotopic ossification, this time in traumatic acetabular fracture patients.