Different anatomic parameters were measured on radiographs of the healthy hip of 47 patients with unilateral slipped capital femoral epiphysis (SCFE). Data were compared with those from 36 hips of 23 healthy adolescents matched in age, weight, and stature. During 5 years of follow-up, eight patients with unilateral SCFE developed slippage of the contralateral side. Patients with unilateral SCFE had a lower physis-diaphysis angle and a larger angle of physeal sloping in the AP view than controls, reflecting a more vertical orientation of the capital femoral physis. In the axial view, control patients had a mean value of the posterior sloping angle of the capital femoral physis of 5 degrees, compared with 12 degrees in unilateral SCFE cases and 18 degrees in patients developing bilateral SCFE. The risk of developing bilateral disease can be predicted by measuring hip morphometric parameters, the most important factor being the posterior sloping of capital femoral physis in the axial view. Prophylactic pinning of the healthy contralateral side should be strongly recommend only in patients with unilateral SCFE showing an axial posterior sloping angle of the physis of over 12 degrees.