The detrimental effects of increased length of stay at the emergency department (ED-LOS) for patient outcome have been sparsely studied in the Swedish setting. Our aim was to further explore the association between ED-LOS and short-term mortality in patients admitted to the EDs of two large University hospitals in Sweden. All adult patients (> 18 years) visiting the ED at the Karolinska University Hospital, Sweden, from 1/1/2010 to 1/1/2015 (n = 639,385) were retrospectively included. Logistic regression analysis was used to determine association between ED-LOS and 7- and 30-day mortality rates. All patients were triaged according to the RETTS-A into different levels of medical urgency and subsequently separated into five quintiles of ED-LOS. Mortality rate was highest in highest triage priority level (7-day mortality 5.24%, and 30-day mortality 9.44%), and decreased by lower triage priority group. For patients with triage priority levels 2-4, prolonged ED-LOS was associated with increased mortality, especially for lowest priority level, OR for priority level 4 and highest quintile of ED-LOS 30-day mortality 1.49 (CI 95% 1.20-1.85). For patients with highest triage priority level the opposite was at hand, with decreasing mortality risk with increasing quintile of ED-LOS for 7-day mortality, and lower mortality for the two highest quintile of ED-LOS for 30-day mortality. In patients not admitted to in-hospital care higher ED-LOS was associated with higher mortality. Our data suggest that increased ED-LOS could be associated with slightly increased short-term mortality in patients with lower clinical urgency and dismissed from the ED.