A 16-year-old male was referred to our hospital in April 2003 due to severe knee joint pain from five months previously. Lymphoblasts were identified in his peripheral blood, resulting in a diagnosis of acute lymphoblastic leukemia (ALL). Bone marrow examination revealed massive necrosis with clusters of lymphoblasts and the bcr/abl fusion gene. Magnetic resonance imaging (MRI) of the knee joint showed low signal intensity on T1-weighted images, and peripheral rim enhancement on Gd-DTPA enhanced fat suppression images, which was compatible with bone marrow necrosis. After the patient achieved complete remission (CR), the knee joint pain has disappeared. He was treated with an allogeneic bone marrow transplantation (BMT) from an HLA-identical unrelated donor and has been in CR for 26 months after the diagnosis of ALL. In the knee joint, the replacement of fatty marrow after BMT has been confirmed with MRI. Hematological malignancies including ALL should be considered in the cases of bone marrow necrosis and adequate treatment may improve necrosis.