Objective: To precisely delineate clinical risk factors for conversion from idiopathic REM sleep behavior disorder (RBD) to Parkinson disease, dementia with Lewy bodies, and multiple system atrophy, in order to enable practical planning and stratification of neuroprotective trials against neurodegenerative synucleinopathy.Methods: In a 10-year prospective cohort, we tested prodromal Parkinson disease markers in 89 patients with idiopathic RBD. With Kaplan-Meier analysis, we calculated risk of neurodegenerative synucleinopathy, and using Cox proportional hazards, tested the ability of prodromal markers to identify patients at higher disease risk. By combining predictive markers, we then designed stratification strategies to optimally select patients for definitive neuroprotective trials.Results: The risk of defined neurodegenerative synucleinopathy was high: 30% developed disease at 3 years, rising to 66% at 7.5 years. Advanced age (hazard ratio [HR] 5 1.07), olfactory loss (HR 5 2.8), abnormal color vision (HR 5 3.1), subtle motor dysfunction (HR 5 3.9), and nonuse of antidepressants (HR 5 3.5) identified higher risk of disease conversion. However, mild cognitive impairment (HR 5 1. REM sleep behavior disorder (RBD) is characterized by loss of REM sleep paralysis, allowing patients to "act out" dreams.