Two families with Gerstmann-Sträussler-Scheinker disease (GSS) are atypical in possessing neocortical neurofibrillary tangles (NFTs), which are few or absent in other kindreds with GSS, in addition to amyloid plaques that react with prion protein (PrP) antibodies and protease-resistant PrP accumulation in the brain. A leucine substitution at PrP codon 102 has been genetically linked to GSS in some families. We examined the PrP gene in these families. A serine for phenylalanine substitution was found at codon 198 in the Indiana patients; arginine for glutamine substitution at codon 217 in the Swedish patients. These mutations in PrP are the first to be associated with the appearance of both PrP amyloid plaques and neocortical NFTs in GSS patients.