Of 44 patients (55 hips) with slipped upper femoral epiphysis treated from 1963 to 1989, 13 (14 hips) developed chondrolysis. Eight hips had chondrolysis at the time of presentation, all in female patients who were either coloured or black and who had moderate or severe slips. The other six hips had persistent pin penetration of the joint; in five of these the pin penetrated the anterosuperior quadrant of the head. Removal of penetrating pins resulted in improvement in pain in all six hips and in the range of movement in four. Chondrolysis did not develop in any of 11 hips with transient intraoperative pin penetration. In hips with chondrolysis maximum joint-space narrowing developed within the first year; improvement in joint space and range of movement continued for up to three years after maximal involvement. At an average follow-up of 13.3 years no patient had pain but five hips were stiff.