Significant radiation-induced abnormalities (aseptic necrosis of the femoral heads, slipped capital femoral epiphysis, radiation-induced sarcoma) were identified in eight of 44 patients aged 16 years or younger at the time of radiotherapy and followed for at least 3 years. The incidence is 18% in the entire group and 25% (8/32) if only patients with radiographs of the hips 3 or more years after therapy are considered. The first evidence of abnormality developed 13 years after irradiation in one patient. The need for long term follow-up of therapeutically irradiated children is stressed.