STUDY OBJECTIVE: We identify factors that define frequent and highly frequent emergency department (ED) users. METHODS: Administrative health care records were used to define less frequent (1 to 6 visits), frequent (7 to 17 visits), and highly frequent (>=18 visits) ED users. Analyses were conducted to determine the most unique demographic, disease, and health care use features of these groups. RESULTS: Frequent users composed 9.9% of all ED visits, whereas highly frequent users composed 3.6% of visits. Compared with less frequent users, frequent users were defined most strongly by their substance abuse challenges and by their many visits to primary care and specialist physicians. Substance abuse also distinguished highly frequent from frequent ED users strongly; 67.3% versus 35.9% of these patient groups were substance abusers, respectively. Also, 70% of highly frequent versus only 17.8% of frequent users had a long history of frequent ED use. Last, highly frequent users did not use other health care services proportionally more than their frequent user counterparts, suggesting that these former patients use EDs as a main source of care. CONCLUSION: This research develops objective thresholds of frequent and highly frequent ED use. Although substance abuse is prominent in both groups, only highly frequent users seem to visit EDs in place of other health care services. Future analyses can investigate these patterns of health care use more closely, including how timely access to primary care affects ED use. Cluster analysis also has value for defining frequent user subgroups who may benefit from different yet equally effective treatment options. [ABSTRACT FROM AUTHOR]