We compared the clinical characteristics of 3 repeat (3R) and 4 repeat (4R) tau-positive cases (tauopathies) presenting as behavior variant frontotemporal dementia (bv-FTD). We identified and retrospectively reviewed demographics and clinical features of patients with pathologically confirmed tau-positive frontotemporal lobar degeneration in a blinded fashion. Those presenting as bv-FTD were divided according to their tau isoform, 3R versus 4R, and compared with age-matched and sex-matched control patients with 4R tauopathies but presenting clinical syndromes other than bv-FTD. Twenty-four cases with tau-positive bv-FTD and 18 4R tau-positive controls were included in the study. Patients with 4R tauopathies had significantly shorter disease duration than patients with 3R tauopathy (median, 6.5 y vs. 9.5 y; P<0.05), despite similar age of disease onset and regardless of whether bv-FTD was the presenting clinical syndrome. Among bv-FTD cases, those with 4R tauopathies were more likely to display behavioral underactivity than those with 3R tauopathy (P=0.03), although 3R and 4R tauopathy patients shared many similar clinical features. In summary, survival in 4R tauopathies seemed independent of the presenting clinical phenotype, and there may be subtle clinical differences between bv-FTD patients with 3R and 4R tauopathies.