Myeloperoxidase (MPO)- and Sudan Black B-not more than 3%-positive, esterase staining-negative, lymphoid, megakaryocyte lineage and erythroid surface marker-negative and electron microscopic platelet peroxidase-negative acute leukemia (AL) was diagnosed as acute undifferentiated leukemia (AUL), and myeloid marker (CD13, CD33), electron microscopic MPO (EMMPO), and DNA analysis of immunoglobulin heavy chain and T cell receptor as well as chemotherapy and its reactivity were examined. Of 239 cases of AL, 10 (4.2%) were AUL, and of these 10 cases, 9 were CD13 or CD33-positive AML-MO (MO) cases. Of 9 cases examined for EMMPO, 4 (44%) were positive, and of 3 cases of MO subjected to DNA analysis, 1 and 1 showed rearrangements of immunoglobulin heavy chain and T cell receptor beta chain, respectively. Of 6 cases of MO on myeloid induction therapy, 1 and 1 showed complete remission (CR) and partial remission (PR), respectively, each having lymphoid genotype, and 4 showed no remission (NR), being 3 of them EMMPO-positive. Of 2 cases on lymphoid induction therapy, 1 and 1 showed CR and NR, respectively, the former being EMMPO-positive MO. BHAC-EM therapy with behenoyl cytosine arabinoside, VP-16 and mitoxantrone performed on 2 cases refractory to any one of both these myeloid and lymphoid induction therapies led to CR in all these 2 cases.