Our objective was to examine whether rapid eye movement (REM) sleep behavior disorder occurs in disproportionally greater frequency in multiple system atrophy (MSA), Parkinson's disease (PD), and dementia with Lewy bodies (DLB), collectively known as the synucleinopathies, compared to other nonsynucleinopathy neurodegenerative disorders. In study 1, we reviewed the clinical records of 398 consecutive patients evaluated at Mayo Clinic Rochester for parkinsonism and/or cognitive impairment. The frequency of suspected and polysomnogram (PSG)-confirmed REM sleep behavior disorder (RBD) among subjects with the synucleinopathies MSA, PD, or DLB was compared to the frequency among subjects with the nonsynucleinopathies Alzheimer's disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), mild cognitive impairment (MCI), primary progressive aphasia (PPA), and posterior cortical atrophy (PCA). In study 2, we reviewed the clinical records of 360 consecutive patients evaluated at Mayo Clinic Jacksonville for parkinsonism and/or cognitive impairment. The frequency of probable RBD among patients with PD and DLB was compared to the frequency among patients with AD and MCI. In study 3, we reviewed the brain biopsy or postmortem autopsy diagnoses of 23 Mayo Clinic Rochester patients who had been clinically examined for possible RBD and a neurodegenerative disorder. In study 1, patients with MSA, PD, or DLB were more likely to have probable and PSG-confirmed RBD compared to subjects with the nonsynucleinopathies (probable RBD 77/120=64% vs. 7/278=3%, p < 0.01; PSG-confirmed RBD 47/120=39% vs. 1/278=0%, p < 0.01). In study 2, patients with PD and DLB were more likely to have probable RBD compared to those with AD and MCI (56% vs. 2%, p < 0.01). In study 3, of the 23 autopsied patients who had been questioned about possible RBD, 10 were clinically diagnosed with RBD. The neuropathologic diagnoses in these 10 included Lewy body disease in nine, and MSA in one. Of the other 13 cases, 12 did not have a history suggesting RBD, and the one case who did had normal electromyographic atonia during REM sleep on PSG and autopsy findings of PSP. Only one of these 13 had a synucleinopathy. The positive predictive values for RBD indicating a synucleinopathy for studies 1-3 were 91.7%, 94.3%, and 100.0%, respectively. Clinically suspected and PSG-proven RBD occurs with disproportionally greater frequency in MSA, PD, and DLB compared to other neurodegenerative disorders. In the setting of degenerative dementia and/or parkinsonism, we hypothesize that RBD is a manifestation of an evolving synucleinopathy.