Fifteen patients developed acute nonlymphocytic leukemia (ANLL) 31 to 182 months following chemotherapy and/or radiotherapy for various malignancies and one non-neoplastic disorder. The ANLL was commonly heralded by a brief preleukemic phase consisting of cytopenias and a variety of morphologic abnormalities. At diagnosis of ANLL, all of the patients had a panmyelosis with variation in the predominant abnormal cell line. Neutrophilic and erythroid abnormalities were most striking in 12 of the patients, megakaryocytic abnormalities predominated in 2 and monocytic abnormalities in 1. Pancytopenia, marked anisopoikilocytosis, normoblastemia, large hypogranular platelets, hypogranular neutrophils, pseudo-Pelger-Huet nuclei, low myeloblast counts and basophilia were the most common abnormalities in the blood. Bone marrows were hypercellular with increased myeloblasts and basophils, abnormal neutrophil precursors, occasional monocytoid blasts, dyserythropoiesis with PAS positive erythroblasts, ring sideroblasts and micromegakaryocytes. All of the 7 patients who had bone marrow chromosome studies exhibited major chromosomal abnormalities. Response to anti-leukemic therapy was poor. The morphologic and clinical findings of these 15 patients appear to define a clinical-pathologic entity.