The effectiveness of a single 6.5 or seven-millimeter-diameter screw for the promotion of premature physeal closure and the provision of stability of a slipped capital femoral epiphysis was investigated. Physeal fusion was demonstrated in forty-nine (92 per cent) of fifty-three hips after fixation with a single screw. Premature fusion of the involved physis, compared with the uninvolved, contralateral physis, was documented (p less than 0.001). Epiphyseal stability, as measured by the lateral head-shaft angle, was maintained in all except one hip. An analysis of twenty-nine hips for which there was a complete set of radiographs, that were in patients who had no endocrine problems, and that were the first hips entered into the study when the patient had bilateral involvement, revealed an average time to closure of the physeal line of thirteen months. A longer time to physeal fusion was correlated with increasingly eccentric placement of the screw (r = 0.44, p = 0.016) and increasing severity of the slip (r = -0.536, p = 0.003). There was no correlation between the age at the time of the operation, race, or sex and the time to physeal fusion. Only one patient had penetration by a screw, and no chondrolysis, avascular necrosis, or other serious problems developed. Fixation with a single 6.5 or seven-millimeter-diameter screw provided adequate epiphyseal stability and promoted premature physeal fusion in our patients, while decreasing the rate of complications compared with that reported to be associated with fixation with multiple screws or pins.