The purpose of the study was to find out the prevalence of osteonecrosis in children with acute lymphoblastic leukemia (ALL) in complete bone marrow remission at the end of the treatment. Twenty-eight children with ALL underwent MRI of the upper and/or lower extremities. Bone marrow signal intensity was analyzed on T1-weighted images, where circumscribed lesions with a rim of low signal intensity were considered typical of osteonecrosis. Osteonecrosis was found in 9 of the 28 children (32%, 95% CI 16% to 52%). Five of them were asymptomatic. They had been treated with high risk and intermediate risk protocols, both of which include a delayed intensification phase with dexamethasone. None of the patients with standard risk ALL were found to have developed osteonecrosis. Osteonecroses occurred unexpectedly in symptomless patients and in patients with mild transient symptoms treated with high risk and intermediate risk protocols. Our study suggests that the intensification phase of the treatment protocols with intensive dexamethasone medication might be responsible for the development of osteonecrosis.