Several agents, including arabinosyl cytosine (ARA-C) at a low concentration, can induce leukemic myeloblasts to mature to a variable extent. The therapeutic implications of this observation are worth investigating. A few case-reports have shown that low dose ARA-C can be useful for treatment of the myelodysplastic syndromes (MDS) and of acute myeloid leukemia (AML). However, no information is available yet on the proportion of patients who can be expected to respond. We treated by low dose ARA-C (20-30 mg/sqm/day i.v. or i.m. for 7-10 days) 20 consecutive patients. A complete remission of 5 months was obtained in one of nine cases of subacute myeloid leukemia (SAML). A partial remission (complete normalization of blood counts with a slight excess of marrow blast cells) was obtained twice in one of 11 cases of MDS. An increase of Hb level (more than 11.5 g/dl) was obtained and maintained for 12 months in a case of MDS. A short-lasting increase of granulocyte count was obtained in another two cases of MDS and SAML respectively. It is suggested that low dose ARA-C can advantageously modify the proliferation to maturation imbalance of leukemic cells by slowing down cell proliferation rate. However, the proportion of patients who respond is probably low. This treatment is at a very early experimental stage and should be probably limited to selected cases of MDS and subacute or acute myeloid leukemia.