Two patients with acute leukemia developed abnormal liver function tests after the administration of cytosine arabinoside. Other possible causes for such abnormalities were not likely. In both patients a close chronologic relationship between the administration of the drug and the appearance of the laboratory abnormalities was noted. The liver damage was also documented by biopsy. To our knowledge this is the first time that the hepatotoxic effects of cytosine arabinoside are histologically demonstrated.