Preleukemia refers to the syndrome of hematopoietic dysfunction seen in some patients before typical features of acute myelogenous leukemia develop. Clinically, preleukemia is characterized by variable degrees of pancytopenia, with associated symptoms due to decreased circulating blood elements. The bone marrow is usually hypercellular, and there are prominent morphologic abnormalities in erythroid precursors and more subtle changes in megakaryocytes and granulocyte progenitors. During this phase of the leukemic process, the neoplastic clone is usually already established and predominant. A clear distinction should be made between the preleukemic syndrome and diseases that predispose to development of acute myelogenous leukemia. Preleukemia, like acute myelogenous leukemia, is a clonal hematopoietic stem cell neoplasm manifested functionally by abnormal hematopoietic cell maturation and ineffective hematopoiesis. During the course of preleukemia, precursor cell maturation becomes progressively impaired with termination in the severe maturational block characteristic of acute myelogenous leukemia. We favor the concept that preleukemia is an early phase of acute myelogenous leukemia.