A few epidemiologic studies are available on the prevalence of early-onset Alzheimer disease (AD) and frontotemporal lobar degeneration (FTLD). The aim of this study was to establish in an Italian population, namely in Brescia County, the prevalence of early-onset neurodegenerative dementia, and how it is shared between AD and FTLD. A network among the participating centers has been established for 10 years. A standardized form was sent to be filled in for each patient. The census day was chosen as December 1, 2009. The prevalence of disease was calculated stratifying patients according to sex and diagnosis. On the census day, 175 patients in the whole population aged 45--65 years were enrolled into the study. The resulting overall prevalence of early-onset neurodegenerative dementia was found of 55.1 per 100,000 inhabitants (95%confidence interval, 47.0--63.4). A comparable prevalence between AD and FTLD was reported (25.5 and 29.6 per 100,000 inhabitants, respectively), and no differences in sex distribution were found both in AD and FTLD. The improvement of knowledge on early-onset neurodegenerative dementias allows us to reconsider its epidemiology and to rethink its impact on public polices. This would be crucial for defining the urgency of treatment approaches.