Genetic factors are involved in the aetiology of dementias. Three genes have been identified which, when mutated, cause Familial Alzheimer disease (FAD): the presenilin-1 (PS1), the presenilin-2 (PS2) and the amyloid precursor protein (APP) genes. Together, these mutations are responsible for 30-50% of the cases with autosomal dominant Alzheimer disease (AD), and for about 5% of all AD cases. While over 130 mutations have been identified in PS1, mutations in PS2 and APP are rarer, since only 10 and 22 mutations, respectively, have been found in these FAD genes. Instead, mutations in the MAPT gene were associated with Familial Frontotemporal dementia (FFTD) linked to chromosome 17 (FTDP-17). Frontotemporal dementia (FTD) can occur in a sporadic form, but in 30-50% of cases there is a positive family history of dementia. In this study, we determined the spectrum of mutations and the relative contribution of the above mentioned four genes in our Italian clinical series of patients with a positive family history of dementia.