Growth of commonly occurring filamentous fungi in foods may result in production of mycotoxins, which can cause a variety of ill effects in humans, from allergic responses to immunosuppression and cancer. According to experts, five kinds of mycotoxins are important in human health around the world: aflatoxins, ochratoxin A, fumonisins, certain trichothecenes, and zearalenone. These toxins are produced by only a few species of fungi, in a limited range of commodities. Aflatoxins are potent carcinogens, produced by Aspergillus flavus and A. parasiticus in peanuts, maize and some other nuts and oilseeds. Ochratoxin A is a kidney toxin and probable carcinogen. It is produced by Penicillium verrucosum in cereal grains in cold climates, by A. carbonarius in grapes, wines and vine fruits, and by A. ochraceus sometimes in coffee beans. Fumonisins, which may cause oesophageal cancer, are formed by Fusarium moniliforme and F. proliferatum, but only in maize. Trichothecenes are highly immunosuppressive and zearalenone causes oestrogenic effects; both are produced by F. graminearum and related species. Current reporting probably underestimates the effect of mycotoxins as a cause of human mortality.