We made a retrospective review of 198 children with cerebral spasticity to determine development of heterotopic ossification. Of 61 children undergoing hip adduction lengthening, 21 had mild to moderate grades of heterotopic ossification. Only two of 132 patients who underwent spine fusions had developed heterotopic ossification, but in both it was severe, resulting in hip fusion. Two of five patients undergoing concomitant hip surgery and spine fusion developed heterotopic ossification of severe degree. The occurrence of heterotopic ossification after hip muscle surgery in children with cerebral spasticity is common but usually minor. Heterotopic ossification developing after spine fusion is rare but severe when it occurs, and concomitant hip and spine surgery causes frequent severe occurrences of heterotopic ossification.