Patient 1; A 78-year-old woman was admitted to our hospital because of general malaise. Her peripheral blood count showed bicytopenia of 900/microliter with 2% blasts, 7.0 g/dl hemoglobin, and 199,000/microliter platelets. Examination of bone marrow revealed hypocellular marrow with peroxidase-negative blasts (89%). Surface marker analysis of blast cells revealed CD13 agents. Electron-microscopically, myeloperoxidase staining was positive. The diagnosis was hypoplastic leukemia in which blasts had a feature of minimally differentiated acute myeloid leukemia (AML-MO). Patient 2; A 78-year-old man was admitted to our emergency unit because of dizziness and dyspnea on exertion. Examination of peripheral blood showed pancytopenia but no blast cells. Examination of bone marrow showed a markedly hypocellular marrow with 38% blast cells. These blast cells were negative for myeloperoxidase and they had CD13 antigen on their surfaces. The diagnosis was minimally differentiated hypoplastic leukemia. At the time of this writing these two patients had been receiving only red cell transfusions for about six months, and the disease had not progressed.