These days it is widely believed by experts that most cancers are caused by environmental factors. The epidemiology of most of these suspected factors is still equivocal. One such factor which has been widely studied is aflatoxin, a chemical produced by the fungus Aspergillus flavus. Because of the widespread distribution of this agent, its known toxicity, and its frequent ingestion by humans, aflatoxin may well be a significant cause of cancer worldwide. This paper attempts to summarize some of the historical and epidemiological evidence regarding aflatoxins and disease.