A multicenter cohort of idiopathic/isolated rapid eye movement sleep behavior disorder (iRBD) was studied to determine any predictive factors for α-synuclein–related neurodegenerative illnesses. At the start of the study, patients with iRBD from 12 different sites were given a complete assessment for potential environmental and lifestyle risk factors using a standardized questionnaire. Patients were then prospectively monitored and assessed for parkinsonism or dementia at the end of the study. The cumulative incidence of parkinsonism or dementia was calculated with competing risk analysis. Over an 11-year follow-up period, Cox regression analyses were conducted to assess the predictive significance of environmental/lifestyle factors, adjusted for age, sex, and center. About 281 patients out of 319 who were free of parkinsonism or dementia submitted follow-up data. After an average of 5.8 years of follow-up, 130 patients (46.3%) developed a neurodegenerative illness. After 3 years, the overall phenoconversion rate was 24.2%, 44.8% after 6 years, and 67.5% after 10 years. Prior pesticide exposure (adjusted hazard ratio [aHR]=0.21–0.64), rural living (aHR=0.53), lipid-lowering medication use (aHR=0.59), and respiratory medication use (aHR=0.36) were associated with lower phenoconversion risk, whereas prior pesticide exposure (aHR=0.21–0.64), rural living (aHR=0.53), lipid-lowering medication use (aHR=0.59), and respiratory medication use (aHR=0.36) were associated with higher Dementia-first converters had lower coffee and beta-blocker intake, as well as a higher frequency of family history of dementia, compared to those converting to primary dementia and parkinsonism. Environmental factors and concomitant disorders have predictive significance in identifying RBD patients at increased risk of phenoconversion, according to the outcomes. CLICK TO REVIEW