We reported a 68-year-old woman with acute nonlymphocytic leukemia, in whom the leukemia transformed from poorly differentiated myeloperoxidase (MPO)-negative type into myelomonocytic type during the observation without chemotherapy. Hematological findings on admission revealed a leukocyte count of 3,500/microliters with 48% blasts and a platelet count of 9.2 x 10(4)/microliters. Bone marrow aspiration showed 68.2% infiltration of blasts negative for MPO. Sudan black B and esterase stains. By electron microscopy MPO was detected in the endoplasmic reticulum and nucleoenvelope of the blasts. Large vacuole-like granules were MPO-negative. She was observed without administration of any antileukemic agent or an immunopotentiator. The leukocyte count rose gradually, in association with increases in the relative and absolute counts of mature neutrophils and monocytic cells, and the platelet count. Twenty-six months after the initial diagnosis, a blood examination showed a leukocyte count of 74,300/microliters with 20.5% mature neutrophils and 15.5% monocytic and a platelet count of 31.4 x 10(4)/microliters. Cytological, cytochemical, ultrastructural and immunological studies of the bone marrow cells showed features compatible with acute myelomonocytic leukemia (FAB M4). This case is unusual in respect that poorly differentiated ANLL transformed spontaneously into moderately differentiated ANLL.