Seven cases of Reye's syndrome in which aflatoxin B1 was isolated from the blood or liver or both are presented. In two cases aflatoxin B1 was found in the blood during the acute phase of the disease; a finding not previously reported. In six cases aflatoxin B1 was recovered from autopsy specimens of liver. A number of case reports linking aflatoxin B1 to Reye's syndrome have appeared in the literature but until now only one case has been reported from the United States. Aflatoxin B1 and its possible role in the etiology of Reye's syndrome is discussed. It is concluded that Reye's syndrome is the result of multiple interrelated factors.