Isolated REM sleep behavior disorder (iRBD) is a prodrome of α-synucleinopathies like Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The best marker for prediction of lead time to phenoconversion is reduced striatal dopamine transporter (DaT)-binding, unavailable for most patients. This study investigated EEG alpha peak frequency (APF) slowing - an established marker of cognitive deterioration - as a severity marker in iRBD patients. In 320 patients clinically suspected of RBD 3-night-polysomnography was performed. After exclusion of 131 patients, mainly due to psychotropic medication, three groups were studied: Non-Syn - motor behavior unrelated to RBD (n = 34); iRBD (n = 122); RBD converted to overt α-synucleinopathies (PD = 19; DLB = 14). Data show in patients with iRBD significant correlations between APF, DaT-binding ratios and cognition. The strong correlation of APF < 8 Hz with caudate DaT-binding (r = 0.50-0.65) suggests, APF could be an easy-to-use severity marker for counseling patients on lead time until possible conversion to overt α-synucleinopathy especially in DLB.