Purpose To analyze the impact of different body mass index (BMI) as a surrogate marker for heterotopic ossification (HO) in patients who underwent surgical repair (SR) for displaced acetabular fractures (DAF) followed by radiation therapy (RT). Methods and Materials This is a single-institution retrospective study of 395 patients. All patients underwent SR for DAF followed by RT ± indomethacin. All patients received postoperative RT, 7 Gy, within 72 h. The patients were separated into four groups based on their BMI: <18.5, 18.5–24.9, 25–29.9, and >30. The end point of this study was to evaluate the efficacy of RT ± indomethacin in preventing HO in patients with different BMI. Results Analysis of BMI showed an increasing incidence of HO with increasing BMI: <18.5, (0%) 0/6 patients; 18.5–24.9 (6%), 6 of 105 patients developed HO; 25–29.9 (19%), 22 of 117; >30 (31%), 51 of 167. Chi-square and multivariate logistic regression analysis showed that the correlation between odds of HO and BMI is significant, p < 0.0001. As the BMI increased, the risk of HO and Brooker Classes 3, 4 HO increased. The risk of developing HO is 1.0× (10%) more likely among those with higher BMI compared with those with lower BMI. For a one-unit increase in BMI the log odds of HO increases by 1.0, 95% CI (1.06–1.14). Chi-square test shows no significant difference among all other factors and HO (e.g., indomethacin, race, gender). Conclusions Despite similar surgical treatment and prophylactic measures (RT ± indomethacin), the risk of HO appears to significantly increase in patients with higher BMI after DAF. Higher single-fraction doses or multiple fractions and/or combination therapy with nonsteroidal inflammatory drugs may be of greater benefit to these patients.