The classical prion diseases in man comprise Creutzfeldt-Jakob disease (CJD), Kuru and Gerstmann-Sträussler-Scheinker syndrome (GSS). Recent advances in the biochemistry and the molecular biology of the transmissible agents responsible for these human spongiform encephalopathies have prompted renewed interest in their clinical and pathological features. A broadening spectrum of human prion diseases has now been identified including novel entities such as Fatal Familial Insomnia and variants of CJD and GSS characterised by specific abnormalities in the human prion protein (PrP) gene on chromosome 20. Accumulation of PrP in the central nervous system is a characteristic feature of all these disorders, although the relationship between PrP localisation, classical neuropathology, clinical features and genotype still requires clarification. A national surveillance project for CJD was established in 1990 in the United Kingdom in order to assess the possible implications of bovine spongiform encephalopathy for human health. The identification of an apparently new variant of CJD in young patients in UK raises the possibility of such a link; further studies are required to assess the significance of this observation.