While [18F]-AV-1451 was developed as a PET radiotracer with high affinity for hyperphosphorylated tau, it has been proposed that loss of 'off-target' [18F]-AV-1451 binding to neuromelanin in the substantia nigra could be a surrogate marker of Lewy body diseases. [18F]-AV-1451 binding was measured in the substantia nigra of patients with Parkinson's disease (n = 35), dementia with Lewy bodies (n = 10) and separate control groups (n = 37; n = 14). Associations with motor symptoms, cognition and disease duration were evaluated using linear regression models. The dementia with Lewy bodies group had significantly reduced substantia nigra [18F]-AV-1451 binding compared to controls after adjusting for age (P < 0.05). However, there were no significant differences in substantia nigra [18F]-AV-1451 binding between Parkinson's disease and controls. Substantia nigra [18F]-AV-1451 binding was not associated with age, disease duration, Movement Disorders Society-Unified Parkinson's Disease Rating Scale and cognitive scores in dementia with Lewy bodies and Parkinson's disease groups. Despite the reduction of substantia nigra [18F]-AV-1451 binding in dementia with Lewy bodies, these findings suggest that substantia nigra [18F]-AV-1451 binding has no value as a diagnostic marker in early Parkinson's disease. Further investigations in longitudinal cohorts are warranted.