The putative involvement of mycotoxins in human diseases, including cancer, is reviewed with reference to ergotism, citreoviridin toxicosis, alimentary toxic aleukia, stachybotryotoxicosis, Balkan endemic nephropathy and aflatoxicoses. Analytical and sampling problems in determining the occurrence of and exposure to mycotoxins are discussed against the background that over 300 mycotoxins have been identified, produced by some 350 fungal species, and that the potency of some of them demands the quantitation of extremely small quantities of analyte.