The incidence of aseptic osteonecroses in the therapy of acute leukaemias in children has been studied. Out of 551 children treated at the Children's Hospital in Münster from 1971 to 1985, 6 developed osteonecrosis, an incidence of 1.09%. Of these children, 5 showed unilateral or bilateral necrosis of the femoral head. The osteonecroses occurred 8-109 months after initiation of the primary therapy or of the relapse treatment. The corticoid doses did not differ from those administered to other leukaemia patients without necrosis. Only 1 patient had received prednisone continuously for 1 year, at a total dose of 20.5 g/m2 of body surface area. Of these 6 children, 4 had been immobilized for several weeks before or during therapy. Two children had presented with pain-related relieving posture of the joints in which subsequently the osteonecrosis developed. Inactivity associated with the cortisone therapy seems to be an important factor in the development of aseptic osteonecroses.