Objective: The length of stay in the emergency department (ED) is a major contributor to ED overcrowding, which in turn negatively affects the quality of emergency care. Several efforts have been made to reduce the ED length of stay (ED-LOS), including a mandatory target to limit ED-LOS within certain parameters. However, the association between ED-LOS and treatment results is yet to be clarified. The authors investigated the influence of ED-LOS on patient survival by comparing severity-adjusted survival. Methods: This study was a retrospective analysis of data registered in 2018 in the National Emergency Department Information System (NEDIS). Cases registered by the regional and local emergency centers were included for analysis. The standardized W scores (Ws) based on the Emergency Department Initial Evaluation Score were used to assess treatment outcomes represented by severity-standardized survival, and the correlation between the Ws and the ED-LOS was analyzed. Results: A total of 2,281,526 cases were included for analysis. The overall mortality comprised 52,284 cases (2.3%) and the median ED-LOS was 165 minutes (interquartile range, 96-301). Although a longer ED-LOS was associated with poorer outcomes overall, the association was not apparent when an analysis of cases eligible for ED-LOS evaluation in the national evaluation program was carried out. Moreover, in the analysis of severe cases with a predicted survival probability of less than 0.9, an ED-LOS shorter than 6 hours was associated with significantly poorer severity-adjusted survival. Conclusion The study revealed that the current ED-LOS criteria used in the national evaluation program were not associated with better survival.