Anaphylaxis is a serious allergic reaction that is rapid in onset and may cause death. Understanding the epidemiology of anaphylaxis has important implications for preventing anaphylaxis and managing patients. Unfortunately, estimates of the prevalence and incidence of anaphylaxis vary widely. Inconsistent definitions of anaphylaxis contribute to these divergent estimates. The implementation of a simple clinical definition to diagnose anaphylaxis not only would help in the management of patients but also would advance epidemiologic research on the distribution and determinants of this potentially life-threatening disease.