In a significant proportion of patients with acute myeloid leukemia (AML), a series of hematological alterations--including refractory anemia, neutropenia, thrombocytopenia, abnormal iron metabolism, and elevated levels of blast cells both in peripheral blood and bone marrow--are observed before the diagnosis of AML is made. This preleukemic state has called the attention of several investigators around the world, since it represents a way to study the origin and progression of leukemia in man. During the past 5 years, major advances in the molecular and cellular biology of this disease have been achieved. It is now known that preleukemia is a clonal disorder that arises from a malignant transformation at the level of primitive pluripotent hemopoietic stem cells. The hemopoietic progenitors in preleukemic patients have abnormal responses to hemopoietic regulators, thus, they do not seem to follow the controlled proliferation observed in the hemopoietic system under normal conditions. The mechanisms of cell differentiation and maturation are also altered, leading to the production of immature (blast) cells, instead of the development of fully mature erythrocytes, granulocytes, platelets and lymphocytes. Several oncogenes, such as C-FMS and RAS, have been found to be structurally altered in a significant proportion of preleukemic patients, suggesting that they may be involved in the pathogenesis of the disease. In spite of the advances made during the last few years, major questions regarding the biology of this hematological disorder are still unanswered.(ABSTRACT TRUNCATED AT 250 WORDS)