Mixed pathologies of α-synuclein, β-amyloid and tau are relatively common in Parkinson's disease (PD) and Dementia with Lewy Bodies (DLB). We therefore wanted to study the retention patterns of 18F-AV-1451 in PD, PD-dementia (PDD), and DLB. To do this 44 healthy controls, 11 non-demented patients with PD, 18 patients with PDD, and six patients with DLB underwent MRI and 18F-AV-1451 PET scanning and cognitive testing. We found that parietal 18F-AV-1451 retention was increased in patients with DLB compared to controls and PD patients, while 18F-AV-1451 uptake was reduced in the substantia nigra in PDD. Increased parietal 18F-AV-1451 PET uptake was associated with impaired performance on verbal fluency tests, and the decreased uptake in the substantia nigra correlated with worse motor function. We found no effect of the monoamine oxidase B inhibitor rasagiline on 18F-AV-1451 binding. In conclusion DLB patients have increased parietal 18F-AV-1451 uptake. Increased parietal tau is associated with executive impairment in patients with synucleinopathies, while decreased uptake in the substantia nigra is associated with parkinsonism. Further, our data indicate that 18F-AV-1451 does not significantly bind to MAO-B in vivo.