Fourteen patients with acute nonlymphocytic leukemia were treated with low-dose arabinosylcytosine (LDAC). Thirteen patients received subcutaneous injections at a dose of 10 mg/M2 every 12 h. One patient received 25 mg intramuscularly daily. All cases received one to three courses with each course lasting 10-60 days (median 19). Complete remission was achieved in 6 (or 43%) of the patients. Three patients had only cytoreduction and 5 patients did not respond. During the therapy severe thrombocytopenia occurred in all patients while prominent other cytopenias occurred in 10. Two-thirds of the patients achieving a remission had significant myelosuppression. There was one treatment-related death. During therapy 11 patients demonstrated a decrease in leukemia cells with an associated increase in differentiated granulocytes. This included 3 of the 4 complete remitters, and 3 of the 5 nonresponders. These results seem to suggest that the therapeutic effect of low-dose Ara-C may result from a combination of differentiation induction, cytotoxicity and unusual sensitivity of the leukemic cells to this agent.