OBJECTIVES: The polysomnographic hallmark of rapid eye movement (REM) sleep behavior disorder (RBD) is loss of muscle atonia during REM sleep; REM sleep without atonia (RSWA). However, the International Classification of Sleep Disorders (ICSD) criteria did not suggest optimized diagnostic value of RSWA and how to score muscle activity during REM sleep. Many previous studies have been tried to figure out the objective quantitative cut-off values of RSWA. We investigated RSWA in Korean RBD patients compared with normal control, to quantify the cut-off value in diagnosis of RBD, using both manual and computer-assisted scoring methods (REM atonia index, RAI). METHODS: We retrospectively analyzed polysomnography and clinical data of 40 patients, 10 age-matched controls by ICSD-3 criteria. The quantitative analysis of chin electromyography density during REM sleep was done by both manual and computerized method. The RSWA and RAI were compared within two groups. RESULTS: In computerized method, the mean RAI of RBD patient was 0.66+/-0.20, and 0.93+/-0.38 in control group (p<0.001). Also, in manual method, RSWA were also significantly increased within RBD patient compared with normal controls (tonic activity: 9.1+/-10.3 vs. 0.1+/-0.2, phasic activity: 7.5+/-6.4 vs. 1.6+/-1.2, p<0.002). CONCLUSIONS: We quantify the characteristics of RSWA in Korean RBD patients and suggest that we may diagnose RBD who present RSWA with 4.1% or more increase in total REM sleep duration and RAI more than 0.84.