The pyrimidine analog, 5-azacytidine (NSC 102816), was administered by continuous intravenous infusion in Ringer's lactate in increasing doses to sets of patients with metastatic cancer to establish a dose sufficient to produce mild toxicity. Twenty-one patients (23 trials) were treated with doses of 50–200 mg/sq/m/day for 5 days every 2–4 wk. Nausea and vomiting were moderate and easily preventable. Doses of 100- 200 mg/sq/m for 5 days every 14 days produced granulocytopenia, usually after two courses. Less toxicity was observed when courses were given every 21–28 days. Forty-five patients with previously treated and refractory acute myeloblastic leukemia were treated. The majority received doses of 150 mg/sq m for 5 days every 2 wk. Eleven (24%) complete remissions and four partial remissions were observed. The number of courses to achieve remission averaged three and required an average of 59 days. Nine patients with blastic crisis of chronic myeloblastic leukemia and four with refractory acute lymphoblastic leukemia failed to respond. 5-Azacytidine administered by continuous infusion is well tolerated and is an active compound in acute myeloblastic leukemia.