Rapid eye movement (REM) sleep behavior disorder (RBD) was described more than 2 decades ago, but only 1 report on 5 patients and 5 normal subjects has tested the effectiveness of a method by which relevant polysomnographic findings can be quantified. We sought to validate this method in a larger sample of patients and control subjects. Cross-sectional. Academic hospital. A clinician interviewed 17 patients at risk for RBD secondary to neurodegenerative disorders and 6 controls to assess whether RBD was present by history. Bed partners completed a questionnaire that quantified RBD symptom severity. From 2 consecutive nocturnal studies in each patient, 2 different polysomnographic RBD scores were generated: the percentage of 30-second REM epochs with at least 15 seconds of tonically maintained electromyographic activity, and the percentage of 3-second REM mini-epochs that contained phasic electromyographic bursts. The tonic and phasic measures, combined together, were higher in patients with clinical determinations of probable or possible RBD (n = 9) than in patients judged unlikely to have RBD (n = 14, P = .023). The overall polysomnographic measure correlated with the symptom scores (rho = 0.42, P = . 048). Specific polysomnographic RBD measures on night 1 correlated highly with those on night 2 (rho > 0.70, P < .0001). This quantitative method to assess the severity of RBD polysomnographic features appears to be both valid and reliable in patients at risk for RBD because of neurodegenerative disorders.