This study was designed to determine whether prophylactic pinning of the unaffected hip in unilateral slipped capital femoral epiphysis affects the proximal femoral morphology. Twenty-four hips prophylactically pinned were compared with 26 cases observed. The articulotrochanteric distance (ATD) and the trochanteric–trochanteric distance (TTD) were measured. Postoperative radiographs were compared with final follow-up radiographs. The final TTD : ATD ratio was higher (P=0.048) in the pinned group, suggesting relative coxa vara/breva. There was a smaller difference between the two hips in the prophylactically pinned group (0.7) as opposed to those observed (1.47). Prophylactic pinning does not cause growth to stop immediately but alters the proximal femoral morphology.