Frontotemporal lobar degeneration (FTLD) refers to heterogeneous clinical and biological conditions. In FTLD, cerebrospinal fluid (CSF) tau levels have been reported highly variable. The aim of the present study was to evaluate whether CSF tau might be the hallmark of a distinct FTLD phenotype. Fifty-five FTLD patients, who underwent CSF analysis, were considered in the present study. In each patient, a wide standardized neuropsychological evaluation, and CSF tau, phospho-tau, and amyloid-β (Aβ) dosages were performed. Each patient was followed-up to five years, and outcomes carefully recorded. In a subgroup of patients (n = 24), magnetic resonance imaging scanning was performed, by using voxel-based morphometry, for grey matter investigation. The higher the CSF tau levels, the worse the neuropsychological and neuroimaging pattern, mainly characterized by greater language disturbances and left temporal grey matter loss. The same pattern, even if less significant, was associated with CSF phospho-tau, while CSF Aβ levels did not play any influence on FTLD phenotype. FTLD patients with high CSF tau showed poor prognosis compared to those with low CSF tau (p = 0.031). In FTLD, CSF tau levels might be considered a marker of neurodegeneration, associated with a specific clinical and neuroimaging picture, and significantly related to poor outcome. Further studies aimed at defining the biological underpinnings of these findings are warranted.