The aims of this study are, first, to determine the incidence of heterotopic ossification (HO) in patients with cerebral palsy (CP) who have undergone pelvic and/or proximal femoral osteotomies and, second, to identify any risk factors that may contribute to its development in this patient population. The radiographs of 219 consecutive patients with CP who underwent proximal femoral osteotomies with or without pelvic osteotomies were reviewed. Risk factors including gender, age, and degree of involvement, ambulatory status, previous hip operations, bilateral hip surgery, capsular release, concomitant pelvic osteotomy, infection, and history of exuberant callus were evaluated. Thirty-five (16%) patients were diagnosed with HO and the 5 factors that cause HO were identified, which are degree of involvement (quadriplegic), ambulatory status, capsular release, infection, and previous hip operations. Based on logistic regression analysis, if a patient had quadriplegic type of CP, then they have 17.5 times more risk for HO than a patient with hemiplegic type, and capsular release increases the risk 237 times. Although HO occurred in 16% of patients treated with bony procedures in the hip, in a small group (2%) of children it had a clinically significant limitation requiring surgical resection. In this study, clear risk factors were presented for the development of HO; however, none of these risk factors can be altered in ways that will reduce the risk for HO. These risk factors might be used to define a high-risk group in whom attempts at prophylactic treatment for prevention of HO could be initiated.