Objectives REM (rapid eye movement) sleep without atonia (RSWA) is a polysomnographic finding used in diagnosis of REM sleep behavior disorder (RBD). Clinical significance of idiopathic RSWA (iRSWA) unaccompanied by RBD is not known. We designed a prospective study to investigate whether iRSWA constitutes an increased risk for developing neurodegenerative disorders. Materials and methods Between January 2010 and December 2014, a total of 4362 patients underwent a full-night video-polysomnography. Upon detailed clinical and polysomnographical examination, patients with iRSWA and idiopathic RBD (iRBD) were enrolled into this study and followed up at every six months for at least 4 years up to 9 years. Results We had a total of 31 patients with iRBD and 67 patients with iRSWA. Mean age was higher in iRBD group than those in iRSWA group (P = .016). Restless legs syndrome/Willis-Ekbom disease was significantly more common in patients with iRBD than those in patient with iRSWA (P < .001). Eighteen patients with iRSWA (26.8%) developed iRBD after 2.6 + 2.2 years. Six patients with iRSWA (8.9%) developed neurodegenerative disorders following 2.4 + 1.5 years; four were diagnosed as Parkinson's disease (PD) and two developed probable Alzheimer-type dementia. In patients with iRBD, eight patients (25.8%) developed neurodegenerative disorders-all was Parkinson's disease-following 2.6 + 2.2 years. Development of neurodegenerative diseases was positively correlated with age (P < .001) and periodic leg movements in sleep in both groups (P < .010). Conclusions These results show that iRSWA may also be accepted as a risk factor in the development of PD or neurodegenerative diseases. Advanced age and periodic leg movements in sleep seem to be correlated with higher risk.