A prospective study of 100 consecutive unilateral fractures of the shaft of the femur was performed to delineate the incidence of, and the factors predisposing to, heterotopic ossification about the hip after intramedullary nailing. Bone debris from reaming of the endosteal canal is deposited in the soft tissues surrounding the site of insertion of the nail, and we postulated that this debris may stimulate the formation of heterotopic bone and that decreasing the amount of debris left in the tissues after nailing may decrease the amount of heterotopic ossification. To test this theory, the patients were treated with routine intramedullary nailing and were randomly divided into two groups. In Group I, the operative incision was irrigated with 250 milliliters of normal saline solution with use of a bulb syringe before the wound was closed, and in Group II, the incision was irrigated with 3000 milliliters of normal saline solution with use of pulsatile lavage. The two groups were similar in all other respects. Eighty patients (eighty fractures; forty in Group I and forty in Group II) were available for follow-up and were evaluated in a blind fashion after the fracture had united. A grading system that was based on the length of the heterotopic ossification, as measured on antero-posterior radiographs of the hip, was used. In thirty-two of the patients (40 per cent), no heterotopic ossification developed, whereas minimum or mild ossification developed in twenty-seven patients (34 per cent). Moderate ossification developed in twelve patients (15 per cent) and severe ossification, in nine patients (11 per cent).(ABSTRACT TRUNCATED AT 250 WORDS)