Retrospective analysis of patients with acute myelomonoblastic (or nonlymphocytic) leukemia has helped to clarify the early stages of the evolution of this disease, and a clinical picture has emerged that is sufficiently discriminating to permit prospective observations. The clinical and laboratory features of the hematologic disorder preceding overt, blast-cell leukemia (i.e., the "preleukemic syndrome" or "hemopoietic dysplasia") are described, and diagnostic criteria and approaches to management are considered. This hematologic syndrome appears to be a stage in a multiphasic panmyelopathy that involves all products of the marrow stem cell. Available data indicate that most of these patients develop acute leukemia within 24 months of the onset of the hematologic abnormalities, but longer courses are not uncommon. Prospective documentation in a large group of patients is needed before the natural course can be accurately defined; such studies are now being planned.