Clinical studies involving 5-azacytidine, a ring analogue of cytidine, began in Europe in 1967 and the United States in 1970, and we review available preclinical and clinical studies here. The drug possesses cytotoxic, antimicrobial, antineoplastic, abortive, and mutagenic activity in various biological systems. 5-Azacytidine is thought to exert its antineoplastic effect through interference with nucleic acid metabolism. The dose-limiting toxicities are nausea, vomiting, and leukopenia, while the incidence of thrombocytopenia is low. Hepatic toxicity ranges from abnormal findings in liver function tests to hepatic coma. Clinical results in solid tumors are not encouraging, but 5-azacytidine shows consistent antitumor activity in patients with acute myelogenous leukemia resistant to previous treatment. An overall response rate of 36%, with 20% complete remissions, was achieved in 200 previously treated patients with acute myelogenous leukemia. Further studies must define the role of 5-azacytidine alone and in combination for the first-line treatment of acute myelogenous leukemia.