One hundred and forty-nine patients (208 hips) were managed for slipped capital femoral epiphysis at The Hospital for Sick Children in Toronto from 1980 through 1990. We retrospectively reviewed the records of twenty-four patients (twenty-six hips) who had an acute slipped capital femoral epiphysis to evaluate the outcome of treatment and possible risk factors for avascular necrosis associated with this condition. The patients were evaluated with the Iowa hip-scoring system, a physical examination, and anteroposterior and lateral radiographs. The severity of degenerative joint disease was assessed on the radiographs with use of the system described by Boyer et al.Eighteen slips were classified as grade 1; seven, as grade 2; and one, as grade 3, according to the system of Southwick. Twenty-three hips were treated with in situ pinning and three, with reduction and pinning. The mean duration of follow-up was 5.9 years (range, 2.1 to 12.8 years). Poor Iowa hip scores and more severe degenerative changes were related to the development of avascular necrosis and to the severity of the slip. Avascular necrosis developed in four hips (15 per cent), two of which had had a reduction. Multiple logistic regression analysis, which included all 150 slips (both acute and chronic) for which there was adequate follow-up, showed that the rate of avascular necrosis was related to both the severity and the acute nature of the slip.