Cerebrospinal fluid (CSF) total Tau levels vary widely in neurodegenerative disorders, thus being not useful in their discrimination over Alzheimer disease. No CSF marker for progressive supranuclear palsy (PSP) is currently available. The aim of this study was to characterise and measure Tau forms in order to verify the differential patterns among neurodegenerative disorders. Seventy-eight patients with neurodegenerative disorders and 26 controls were included in the study. Each patient underwent a standardised clinical and neuropsychological evaluation, MRI, and CSF total-Tau and phospho-Tau dosage. In CSF and cerebral cortex, a quantitative immunoprecipitation was developed. An extended (55 kDa), and a truncated (33 kDa) forms of Tau were recognised. CSF samples were assayed, the optical density of the two Tau forms was measured, and the ratio calculated (Tau ratio, 33 kDa/55 kDa forms). Tau ratio 33 kDa/55 kDa was significantly decreased in patients with PSP (0.46+/-0.16) when compared to controls, including healthy subjects (1.16+/-0.46, P=0.002) and Alzheimer disease (1.38+/-0.68, P<0.001), and when compared to frontotemporal dementia (0.98+/-0.30, P=0.008) or corticobasal degeneration syndrome (0.98+/-0.48, P=0.02). Moreover, in PSP patients Tau form ratio was lower than in other neurodegenerative extrapyramidal disorders, such as Parkinson disease (1.16+/-0.26, P=0.002) and dementia with lewy bodies (1.44+/-0.48, P<0.001). Tau ratio 33 kDa/55 kDa did not correlate either with demographic characteristics, cognitive performances or with motor impairment severity. Truncated Tau production shows a different pattern in PSP compared to other neurodegenerative disorders, supporting the view of disease-specific pathological pathways. These findings are promising in suggesting the identification of a marker for PSP diagnosis in clinical practice.