Synaptic loss is prominent in several human neurodegenerative diseases. We tested the hypothesis that synaptic density is reduced by the primary tauopathies of progressive supranuclear palsy (PSP-Richardson's syndrome) and corticobasal syndrome (CBS). Thirtyseven participants (12 CBS, 10 PSP, and 15 age-/sex-/education-matched controls) underwent clinical and neuropsychological assessment, 3T magnetic resonance imaging, and positron emission tomography with the radioligand [ 11 C]UCB-J which targets the Synaptic Vesicle Glycoprotein 2A (SV2A). 10 CBS patients had negative β -amyloid biomarkers (Pittsburgh Compound B PET). As expected, patients with PSP-Richardson's syndrome and amyloidnegative CBS were impaired in executive, memory and visuospatial tasks. [ 11 C]UCB-J binding was reduced across frontal, temporal, parietal, and occipital lobes, cingulate, hippocampus, insula, amygdala and subcortical structures in both PSP and CBS patients compared to controls (p<0.001), with reductions up to 50%, consistent with post mortem data. The revised Addenbrooke's Cognitive Examination score correlated positively with [ 11 C]UCB-J binding in frontal, temporal, parietal, occipital, and cingulate cortices, as well as in the hippocampus, insula and amygdala, p<0.05); putamen and frontal lobe [ 11 C]UCB-J binding correlated inversely with the PSP rating scale ( p<0.05). In conclusion, we confirm severe synaptic loss in PSP and CBS, which correlates with disease severity, providing critical insights into the underlying pathophysiology of primary degenerative tauopathies and supporting potential treatment strategies based on synaptic maintenance or restoration. Keywords: Synaptic Vesicle Protein 2A, UCB-J, PET, tauopathy, PSP. Abbreviations: [ 11 C]UCBJ = (R)-1-((3-(methyl-11C)pyridin-4-yl)methyl)-4-(3,4,5trifluorophenyl)pyr-rolidin-2-one); BP ND = non-displaceable binding potential; PSP = Progressive Supranuclear Palsy, CBS = corticobasal syndrome, CBD = Corticobasal Degeneration; [ 11 C]PiB = (methyl-11C) Pittsburgh compound B; UPDRS = Unified Parkinson's Disease Rating Scale; PSPRS = Progressive Supranuclear Gaze Palsy Rating Scale; ACE-R = Addenbrooke's Cognitive Examination -Revised; MMSE = Mini-mental State Examination; CDR = Clinical Dementia Rating Scale; CBI = Cambridge Behavioural Inventory -Revised; SEADL = Schwab and England Activities of Daily Living Scale. ns = non-significant at p<0.05.