To develop a polysomnographic video-based scale for rating the severity of REM sleep behavior disorder (RBD), to classify the severity of RBD and to determine the intraindividual variability of RBD in patients with Parkinson disease (PD). Twenty PD patients identified with RBD were investigated with video-supported polysomnography (PSG). Seventy-three motor behavior events during REM sleep were graded visually and polysomnographically on an event-to-event basis according to categorical location of movements: "0" = no visible movement; "1" = slight movements or jerks "2" = movements involving proximal extremities, including violent behavior; "3" = axial involvement including bed falls. Vocalizations were rated as "1" for present or "0" for absent. Ratings were performed by 2 blinded raters. Reliability was calculated with Cohen's κ. Final RBD severity was determined by the highest score given. This rating scale was then used to compare RBD severity and density, calculated as RBD episodes per REM sleep minute over 2 consecutive nights in 10 additional PD patients with RBD. Statistical significance was determined by effect size (Hedges' g) and calculation of the confidence interval. Interrater reliability of the scale was 0.8 for movement data and 0.89 for vocalization data. Intraindividual RBD density varied significantly (effect size 0.5 ± 0.22; confidence interval 0.2 to 0.79) by factor 2.5 between the 2 PSG nights. Final RBD severity score differed in 60% of patients between nights 1 and 2. Forty percent of patients showed violent behavior, but only on one night. All patients had severely disturbed sleep with reduced sleep efficiency, loss of slow wave sleep, sleep fragmentation, and an increased periodic limb movement (PLM) index. The RBD severity scale (RBDSS) is a reliable, easy-to-use tool for assessing motor events during REM sleep with PSG. Severity and phenomenology of RBD shows a significant variability in the individual PD patient.