Fatal familial insomnia has recently enlarged the group of prion diseases. The disease starts between 35 and 60 years of age, is inherited as an autosomic dominant trait, and leads to death within 7 to 32 months. Clinical symptoms and signs include insomnia dysautonomia, cognitive and motor alteration. The discrete topography of the lesions in fatal familial insomnia underlines the role of the thalamus in the regulation of the sleep-wake cycle. Atrophy, neuronal loss and gliosis are prominent in the anterior and dorsomedial nuclei of the thalamus. Spongiosis, which is usually found in prion diseases, is absent in fatal familial insomnia. An abnormal prion protein (PrPsc) is detected in the brain. There is a mutation at codon 178 of the gene encoding this protein. Fatal insomnia is distinct from Creutzfeldt-Jakob disease on clinical, histopathologic and molecular grounds. It provides new information about genetics of prion diseases which share the characteristics of being altogether inherited and, in most cases, transmissible. The recent finding of abnormal PrP in diffuse subcortical gliosis suggests that other degenerative disorders could actually be prion diseases.