A 57-year-old woman presented with a 3-month history of cognitive impairment, daytime somnolence, and violent sleep behavior. Her first- and second-degree relatives had similar symptoms prior to their premature deaths. Her MRI scan of the brain showed no significant abnormality. Electroencephalogram showed loss of slow-wave activity. Functional brain imaging performed with 18F-FDG PET was fused with her MRI scans. This demonstrated profound hypometabolism in bilateral thalami and the posterior cingulate cortex, which is pathognomonic for familial fatal insomnia. Hypometabolism in the temporal lobes suggests a long-standing course of the disease. Genetic testing confirmed a mutation of the prion-protein gene (PRNP).