The aim of the study was to explore the clinical significance of periodic leg movements during sleep (PLMS) in rapid eye movement sleep behavior disorder (RBD) and the pathological relation between these two disorders. Eighty-one consecutive idiopathic RBD (iRBD) patients, classified into two groups-27 patients with PLMS (iRBD-PLMS) and 54 patients without PLMS (iRBD w/o PLMS), and 31 patients with idiopathic PLMS (iPLMS)-were enrolled in this study. Descriptive variables including Epworth Sleepiness Scale (ESS) scores and polysomnography measures were compared among the three patient groups. Correlation analysis between the ratio of PLMS-related arousal index to PLMS index (PLMAI/PLMI) and sleep stage-related variables or clinically descriptive RBD variables was performed in the iRBD-PLMS group. Associated factors indicating the existence of PLMS during both stages NREM and REM were investigated in this group with clinically descriptive RBD variables. The iRBD-PLMS group showed a significantly lower ESS score and PLMAI/PLMI than the iPLMS group. The PLMAI/PLMI value negatively correlated with RWA/REM. RWA/REM was extracted as a factor that was significantly associated with the existence of PLMS during both stages NREM and REM. The RBD morbidity duration appeared as an associated factor for PLMS only during stage REM among the iRBD patients. In iRBD patients, daytime sleepiness remains modest probably because of suppressed cortical reactivity to PLMS. Increased PLMS activity during both stages NREM and REM is related to the mechanism of REM atonia loss caused by brainstem dysfunction. Especially, PLMS during stage REM might reflect the length of RBD morbidity.