At autopsy, frontotemporal dementia (FTD) account for up to 20% of degenerative dementia cases, although FTDs are underrecognized in memory clinics. FTDs are confused with Alzheimer disease (AD) or vascular dementia (VaD). These misdiagnosis may affect the results of AD pharmacological trials. The first manifestations of FTD are behavioral abnormalities. The aim of this study was to assess a behavioral scale of frontal lobe dysfunction and to determine a behavioral cutoff to diagnose early FTD and distinguish it from AD and VaD. The score of the behavioral frontotemporal lobe dysfunction assessment scale was higher in FTD than in other dementias (p < 0.0001). With a cutoff of 3 points on the scale, FTD patients were diagnosed with a specificity of 95% and sensitivity of 91%. Noncognitive symptoms known to be institutionalization factors could contribute to differences between etiologies of mild dementia.