Massive bone marrow necrosis was seen in a 42-year-old male with acute leukemia. In December, 1988, on admission, laboratory data revealed pancytopenia and a high level of serum LDH and ALKP. Bone marrow aspiration resulted in dry-tap and showed bone marrow necrosis in the bone marrow biopsy specimen. A bone marrow scintigraphy with 111In faintly visualized the bone marrow but visualized area was expanded in the extremities compared with normal subjects. The second bone marrow biopsy showed proliferation of blasts. In the middle of March, blasts began to appear in peripheral blood. The blasts were cytochemically negative for POX, Es, PAS, AcP, TdT and had surface markers CD3-, CD19-, CD33-, CD13-, LCA-, HLA-DR-. Even by investigation on rearrangement of the immunoglobulin heavy chain region, an origin of the blasts could not be determined. In April, the number of blasts in peripheral blood increased and hepatosplenomegaly developed rapidly. Therefore, he was put on the chemotherapy with vincristine and prednisolone, but he died of cerebral hemorrhage. The autopsy revealed widespread bone marrow necrosis. It has rarely been reported that massive bone marrow necrosis is found prior to the occurrence of acute unclassified leukemia.