Mutations in the progranulin gene (PGRN) are a major cause of frontotemporal lobar degeneration (FTLD). Herein we estimated the contribution of the PGRN Leu271LeufsX10 mutation to FTLD and related disorders in the Brescia cohort. The PGRN Leu271LeufsX10 mutation was found in 31% of corticobasal syndrome (CBS), 29% of frontotemporal dementia with motorneuron disease (FTD-MND), 15% of behavioral variant frontotemporal dementia (FTD), 9.5% of primary progressive aphasia (PPA), 2% dementia with Lewy bodies and 0% of progressive supranuclear palsy and multiple system atrophy cases. The prevalence strongly increased in familial forms (75% CBS, 50% FTD-MND, 27% FTD, 18% PPA): in our cohort this mutation is a major disease determinant for FTLD-related disorders with a prominent motor component. MAPT haplotype was demonstrated to be a disease modifier in PGRN Leu271LeufsX10 carriers: in H1H2 subjects the disease onset was earlier than in H2H2 individuals. Sequencing of the whole PGRN gene disclosed a previously described mutation (c.2T>C, Met1X) and three novel ones (c.709-3; c.1011delG, His340ThrfsX21; c.1021C>T, Gln341X) in single families. In the Brescia cohort, while MAPT mutations have low prevalence, mutations in PGRN were shown in 28% of familial FTLD and 75% of familial CBS cases. The PGRN Leu271LeufsX10 mutation becomes one of the most common mutations worldwide, since it was identified in 38 patients belonging to 27 unrelated families.