N4-Palmitoyl-1-beta-D-arabinofuranosylcytosine (PLAC) was administered PO to 76 patients with acute leukemia, myelodysplastic syndromes (MDSs), and myeloproliferative disorders (MPDs). Of 20 patients with acute myelogenous leukemia, 2 achieved complete remission, and the only patient with acute lymphoblastic leukemia achieved partial remission. Remission was reached with PLAC 100-300 mg/day 25-66 days after the start of therapy. Among 22 patients with MDS, 1 patient achieved a good response and 8 achieved partial response. Responses were reached with PLAC 50-200 mg/day 7-153 days (median, 33 days) after the start of therapy. Improvement of polycythemia was observed in all 5 patients with polycythemia vera, and reduction of thrombocytosis was observed in 5 out of 6 patients with essential thrombocythemia and myelofibrosis. An antileukemia effect was noted in 1 of 5 with chronic myelogenous leukemia. Major side effects were gastrointestinal toxicities and myelosuppression. In spite of the disadvantages, such as unpredictable absorption and a lower response rate to acute leukemia compared with its parent compound, this antileukemia Ara-C analogue that is administrable PO will be useful in the treatment of MDSs and MPDs, which do not necessarily require admission to hospital, and in the treatment of acute leukemia of the aged, a condition for which intensive chemotherapy is not appropriate.