Thirty-five patients who fulfilled the FAB diagnosis criteria of chronic myelomonocytic leukemia (CMML), i.e., myelodysplastic features, monocytosis over 10(9)/liter, bone marrow monocyte infiltration, blast cells less than 5% in the peripheral blood and less than 30% in the bone marrow, are analyzed. CMML appears as an entity distinct from myelodysplastic and myeloproliferative disorders. Splenomegaly, anemia, thrombocytopenia, leukocytosis with monocytes and granulocytic cells in all stages of development, increased blood and urine lysozyme levels without renal failure, and polyclonal hyperimmunoglobulinemia are its main clinical and biologic features. With conventional cytotoxic drugs (6-mercaptopurine, hydroxyurea), the prognosis of CMML appears poor (median survival 475 days). None of the clinical hematologic or biologic parameters tested had a significant effect on prognosis. As other chemotherapy trials seemed necessary, we recently administered small doses of cytosine-arabinoside (ARA-C) to six patients over several consecutive days and obtained a complete remission in four. These preliminary results must be confirmed by larger series using the diagnostic criteria proposed by the FAB cooperative group.