The prevalence of fronto-temporal dementia may be estimated in 10-20% of all dementia's. In 40 to 50% of the cases it is possible to detect familial antecedents and in some of the families it is possible to identify multiple affected individuals. The study of these familial cases has allowed to narrow the responsible genomic region to chromosome 17 and to identify mutations in the tau gene as responsible for the disease. At present more than 100 cases of the disease due to mutations in the tau gene have been described in which it has been possible to detect 24 different mutations. In order to search for mutations in the tau gene in a patient all it is needed is a sample of venous blood with which it is possible to extract the genomic DNA, amplify the tau gene through PCR and to sequence it. The information obtained provides a very precise diagnosis in patients with fronto-temporal dementia, may be useful in the differential diagnosis with other types of dementia, and may allow a pre-symptomatic diagnosis in relatives from patients. At a more basic level, the detection of mutations in the tau gene allows to identify the pathogenic mechanisms involved opening up the possibility to develop new therapeutics strategies.