There are many challenges for determining the prevalence and incidence of frontotemporal lobar degenerations (FTLD). Consequently, the number of cases of behavioral variant frontotemporal dementia (bvFTD) or primary progressive aphasia (PPA) in the USA is unknown. Our objective was to derive a consensus estimate of bvFTD and PPA prevalence and thereby to estimate the total number of these syndromes in the USA. We identified five prevalence and three incidence studies of FTLD based on passive surveillance and seven studies of survival in FTLD. Data from these studies were used to estimate the number of cases of PPA or bvFTD in the USA. Because prevalence and incidence estimates outside of the 45-64-year age range were either not available or widely divergent, we used data from clinical and pathological series to estimate the proportion of FTLD cases aged <45 or >64 years. The prevalence estimates in the age categories of 45-64 years old have ranged from 15 to 22 per 100,000 person-years in studies where both bvFTD and PPA were identified. The incidence estimates for the same age group ranged from 2.7 to 4.1 per 100,000 person-years. Using a survival rate of 6 to 9 years from onset and rates from the incidence studies, a calculated prevalence estimate (prevalence = incidence × duration) was similar to the previously reported prevalence rates. We estimated that 10% of cases were less than age 45 years and 30% were 65 years and older. We estimate that there are approximately 20,000 to 30,000 cases of the cognitive syndromes of FTLD in the USA. The main threat to the accuracy of the estimates is the difficulty in diagnosing the clinical syndromes that comprise the FTLD group of disorders.