Epidemiological and clinicopathological studies, allied with pathological prion protein (PrPSc) analysis, strongly support the hypothesis that the human prion disease new variant Creutzfeldt-Jakob disease (vCJD) is causally related to bovine spongiform encephalopathy (BSE)1,2, but considerable controversy remains. Distinct prion strains are distinguished by their biological properties on transmission to laboratory animals and by physical and chemical differences in PrPSc strains. We now find that the biological and molecular transmission characteristics of vCJD are consistent with it being the human counterpart of BSE.