The first case of human systemic infection by an Aspergillus flavus isolate demonstrated to produce aflatoxins in vitro and in vivo is described. The patient, a 41-year-old man with acute myelogenous leukaemia, developed a complication of suspected pulmonary Aspergillus infection during remission induction therapy. Antifungal chemotherapy brought about a considerable degree of improvement, but remission of the underlying disease was not attained. Bone marrow transplantation was also not effective. The patient showed recovery from neutropenia but died despite aggresive antifungal chemotherapy. The autopsy revealed lesions in the lungs, myocardium, kidneys, brain, thyroid gland and skin due to a suspected Aspergillus sp. A fungus isolated from the right lung and the skin lesions was identified as A. flavus. Aflatoxins B1, B2 and M1 were detected in culture filtrates of the isolated A. flavus, and in an extract of lung lesions. These aflatoxins are considered to have played an important role in damaging the immune system of the patient through their toxic effects.