BACKGROUND: Emergency department (ED) frequent users account for a disproportionately high number of ED visits. Frequent users are a highly vulnerable subset of ED users, with a significant burden of chronic medical diseases, psychiatric conditions and substance use disorders. Frequent users have an elevated mortality risk compared to the general population and non-frequent users of the ED. However, little is known about risk factors for mortality among frequent users. Our aim was thus to characterize frequent users who died during a randomized trial and to determine predictors of mortality among frequent ED users.