In 25 patients under treatment or during follow-up for acute lymphoblastic leukemia magnetic resonance imaging (MRI) of both knees was compared to history and physical examination to detect osteonecrosis. Results of MRI were classified as: 0, normal bone marrow (BM); 1, diffuse decrease in signal intensity; 2, circumscript lesions. Patients clinically suspected of osteonecrosis had a statistically significant increased incidence of Type 2 lesions. Three patients with a BM relapse also showed Type 2 lesions. Circumscript lesions of the epiphyses were seen exclusively in the four patients with clinical suspicion of osteonecrosis during B M remission and this may be a MR finding strongly indicative of osteonecrosis. Patients with Type 2 lesions during BM remission had received systemic chemotherapy for a statistically significant longer period than the other patients. Using MRI findings we suspect that at least four patients suffered osteonecrosis due to chemotherapy. MRI is a promising means of detecting this disorder.Cancer 60:2994-3000. 1987.HILDREN with acute lymphoblastic leukemia C (ALL) treated with combined chemotherapy sometimes complain of bone or joint pain. This may be due to osteonecrosis. In the literature this is often referred to as avascular necrosis of bone. Many case reports of this complication in patients with leukemia have been published.'-I6 However, the frequency of osteonecrosis in children with ALL is unknown. The reported percentages vary from 3% to 85% depending on the population studied and the time and method of diagnosis. 17-20