To describe a new dementia phenotype of Gerstmann-Straussler-Scheinker disease (GSS) in a previously unreported Italian family. Design Longitudinal clinical and neuropsychological assessment, combined with magnetic resonance imaging (MRI), single positron emission tomography (SPECT) and molecular genetic studies. Setting Neuropsychology Laboratory, and Division of Neuropathology and Neurology, "C. Besta" National Neurological Institute. Patients and participants Three members of the family. Measurements and results. Standardised neuropsychological tests were used to determine cognitive patterns. The proband had a history of primary dementia characterised by loss of initiative, planning, behaviour control, awareness, language and memory; the two relatives suffered from progressive ataxia. MRI of the demented patient revealed brain atrophy with a hyperintense signal in the frontal cortex; SPECT revealed decreased perfusion in the left temporal and parietal cortex and left thalamus. The two ataxic patients showed cerebellar atrophy with no signs of altered perfusion. Analysis of the PRNP gene showed a proline/leucine substitution at codon 102 in all three patients, associated with methionine/valine heterozygosity at the polymorphic codon 129. Conclusions Primary dementia with prominent frontotemporal signs is a new phenotypical expression of P102L-related GSS that coexists in the same family with the ataxic form of the disease. This remarkable variability suggests that still unidentified genetic or acquired factors may modulate the manifestations of GSS. Genetic examination of the PRNP should be included in the diagnostic work-up of patients with poorly classifiable dementia.