Objective: Rapid eye movement sleep behavior disorder (RBD) is associated with neurodegenerative disease and particularly with the synucleinopathies. Convenience samples involving subjects with idiopathic RBD have suggested an increased risk of incident mild cognitive impairment (MCI), dementia (usually dementia with Lewy bodies), and Parkinson disease (PD). There are no data on such risks in a population-based sample. Methods: Cognitively normal subjects aged 70 to 89 years in a population-based study of aging who screened positive for probable RBD using the Mayo Sleep Questionnaire were followed at 15-month intervals. In a Cox proportional hazards model, we measured the risk of developing MCI, dementia, and PD among the exposed (probable RBD [pRBD] +) and unexposed (pRBD -) cohorts. Results: Forty-four subjects with pRBD + status at enrollment (median duration of pRBD features was 7.5 years) and 607 pRBD - subjects were followed prospectively for a median of 3.8 years. Fourteen of the pRBD + subjects developed MCI, and 1 developed PD (15/44 = 34% developed MCI/PD); none developed dementia. After adjustment for age, sex, education, and medical comorbidity, pRBD + subjects were at increased risk of MCI/PD (hazard ratio [HR], 2.2; 95% confidence interval [CI], 1.3-3.9; p = 0.005). Inclusion of subjects who withdrew from the study produced similar results, as did exclusion of subjects with medication-associated RBD. Duration of pRBD symptoms did not predict the development of MCI/PD (HR, 1.05 per 10 years; 95% CI, 0.84-1.3; p = 0.68). Interpretation: In this population-based cohort study, we observed that pRBD confers a 2.2-fold increased risk of developing MCI/PD over 4 years.