We evaluated the prevalence of slipped capital femoral epiphysis in the contralateral hip of 169 children who had been managed with pinning in situ and thirty who had been managed with immobilization in a spica cast. Only children who had initially been seen with a unilateral slip and had been followed for a minimum of two years or until skeletal maturity were included in the study. The average duration of follow-up was 3.6 years (range, 0.5 to 9.5 years) for the group that had been managed with a cast and 2.8 years (range, 1.0 to 8.3 years) for the group that had been managed operatively. In sixty-one (36 per cent) of the 169 patients who had had operative treatment and two (7 per cent) of the thirty who had been managed with a spica cast, a slip subsequently developed in the contralateral hip; this difference was significant (p = 0.001). On the basis of these findings, we recommend that closer attention be paid in the potential development of a slip in the contralateral hip after pinning.