Contamination of corn with the fungus Fusarium moniliforme and its secondary metabolites, the fumonisins, has been associated with several human and animal diseases. This paper summarizes present knowledge and presents new data on the levels of fumonisins present in foods and feeds associated with these diseases as well as in commercial corn and corn-based products. The doses of fumonisins to which humans and animals consuming these products would be exposed are compared with those doses known to produce LEM in horses and hepatocarcinogenesis in rats. It is concluded that the known naturally occurring levels of fumonisins present a potential threat to human and animal health and realistic tolerance levels need to be set.