During the last ten years the diseases scrapie in sheep and bovine spongiform encephalopathy (or mad cow disease) in cattle have received increased attention. Through the 1960s it became apparent that scrapie in sheep and kuru and later Creutzfeldt-Jakob disease in man were infectious diseases. During the last decade the appearance of mad cow disease in Great Britain has increased the fear that humans can develop Creutzfeldt-Jakob disease through their food. A special characteristic of the infectious agent, the prion, is its lack of DNA. It has been shown that the prion most probably has a pure protein structure. The prion may exist in two slightly different structural patterns, one of which induces the various prion diseases in animals and man. In 1994 two men with Creutzfeldt-Jakob disease were diagnosed at Haukeland Hospital. Their symptoms and clinical data are presented, together with a review of the literature on prion diseases in animals and man.