We treated 98 consecutive patients with Perthes' disease by a unilateral brace in external rotation, flexion and abduction and a further consecutive 110 by a bilateral cast with the hips in internal rotation and abduction. During treatment in the unilateral brace, six (6.1%) hips on the opposite side developed evidence of Perthes' disease and one developed this after the brace had been removed. In children managed in bilateral casts, no contralateral Perthes' disease was seen. Adequate containment of the femoral head may prevent subsequent changes in the opposite hip.