Many frequent emergency department (ED) users do not sustain high use over time, which makes it difficult to create targeted interventions to address their health needs. We performed a retrospective analysis of nonelderly adult frequent ED users in California to measure the persistence of frequent ED use in the period 2005–15, describe characteristics of persistent and nonpersistent frequent users, and identify predictors of persistent frequent use. Of the frequent ED users in 2005, 30.5 percent remained frequent users in 2006. A small but nontrivial population (16.5 percent, 5.7 percent, and 1.9 percent) exhibited persistent frequent use for three, six, and eleven consecutive years, respectively. The strongest predictor of persistent frequent ED use was the intensity of ED use in the baseline study year. The rate at which frequent users stopped using the ED frequently decreased over time, leaving a core group of chronic persistent users. These persistent frequent users differ from nonpersistent frequent users, who engaged in temporary intense use of the ED. Identifying and differentiating persistent frequent users is important, as they may be candidates for distinct interventions.