The morphologic and clinical features of four patients who developed significant bone marrow and blood dyspoiesis after successful chemotherapy for acute nonlymphocytic leukemia (ANLL) are described. This postleukemic dyspoiesis developed 1-6 months after leukemia induction therapy and persisted for 5-20 months in a relatively stable state. This period of prolonged dyspoiesis was not associated with rising myeloblast counts or clinical evidence of relapse. Dyspoietic abnormalities developed while two patients were receiving maintenance chemotherapy; the other two patients received no maintenance therapy. The dyspoietic changes in these four patients greatly exceeded those noted in a control group of ANLL patients on maintenance chemotherapy. The morphologic features of postleukemic dysmyelopoiesis were similar to those described in preleukemic dysmyelopoietic disorders. Erythroid abnormalities included hyperplasia with ring sideroblasts, megaloblastic changes, and cytoplasmic PAS reactivity. Myeloid abnormalities consisted of left-shifted granulopoiesis with hyper- and hyposegmentation; megakaryocytic abnormalities included hyperplasia with a predominance of hypolobulated forms. Three of the four patients eventually suffered relapse and have died. The fourth patient died of sepsis after 20 months of pancytopenia and dysmyelopoiesis. Theories to explain the development of postleukemic dysmyelopoiesis are presented which emphasize the possibility of drug-induced leukemia cell differentiation. Cytogenetic studies will be necessary to establish any relationship between ANLL and the subsequent postleukemic dysmyelopoiesis.