Recently it was shown by several research groups that mutations in the gene encoding for the tau protein associated with microtubuli on chromosome 17 caused a distinct form of dementia named frontotemporal dementia and parkinsonism (FTDP-17). This disease includes familial asymmetrical frontal and, in the further course, frontotemporal dementia, parkinsonism, which is often initially sensitive to levodopa, signs of upper motor neuron degeneration, and, less commonly, amyotrophy. Tau is an intracellular protein of the cytoskeleton, which is responsible for the arrangement and stabilization of microtubuli. The discovery of mutations in the tau gene causing a distinct neurodegenerative disease in humans has firmly established the importance of the tau gene for neurodegenerative processes, not only in tauopathies but also in other degenerative disorders with tau pathology, such as corticobasal degeneration, supranuclear progressive paralysis, amyotropic lateral sclerosis, parkinsonism-dementia complex of Guam, and Alzheimer's disease. Our experience with patients suffering from PTDP-17 shows that its phenotype varies more than was described in the first consensus conferences. In the future, it will be important to designate the diagnostic gold standard not by clinical description, but etiologic classification.