OBJECTIVES: To examine characteristics of US patients with emergency department (ED) visits or hospitalization for anaphylaxis, and the cost drivers of these visits.  METHODS: This retrospective study utilized the MarketScan® claims databases to identify patients enrolled in employer-sponsored health plans and visited EDs or hospital with anaphylaxis between 2002 and 2008.  Patients had to have ≥1 year continuous health coverage pre- and post-index (first observed event date) and were excluded if immunocompromised, had pre-index cancer diagnoses or sepsis at index. Descriptive statistics were evaluated for patient characteristics during the pre-index period and for costs of index events. Multivariate analyses were conducted examining cost drivers of index events.  RESULTS: A total of 11,972 patients were identified (mean age 42, children 19%; female 59%; Northeast 12%; triggers: unknown cause 66%, food 20%, venom 8%, medicine 6%), and the majority of events occurred in ED (78%). Compared to adult patients, children (<18 years) were more often male (56 vs. 38%), and had different pre-index comorbidities (4 vs. 1 of 5 most prevalent comorbidities were allergy-related). Mean index event costs were $1553 ± 1705 for ED visits and $14262 ± 36108 for hospital stays. Unadjusted ED costs for children did not differ across triggers (P=0.78), but hospitalization costs did (P<0.001). For adults both ED and hospital event costs differed by trigger (P<0.001). In multivariable analyses unknown and venom triggered event costs for adults were 1.13 (P=0.03) and 1.34 (P=0.01) times greater than those for children regardless of place of service.  Overall, pre-index epinephrine auto-injection device (EAID) fill was associated with lower event cost (0.89 times; 95% CI 0.83-0.95).  CONCLUSIONS: This study found significant differences in clinical characteristics and costs by age group, trigger, and treatment setting among patients with anaphylaxis. Pre-index EAID fill may be associated with lower ED/hospitalization cost, but needs confirmation in future studies.