BACKGROUND: The incidence of errors and adverse events in emergency medicine is poorly characterized. OBJECTIVE: The objective was to systematically determine the rates and types of errors and adverse events in an academic, tertiary care emergency department (ED). METHODS: Prospective data were collected on all patients presenting to a tertiary-care academic medical center ED with an annual census of 55,000 patients between January 2009 and November 2012. Cases of patients meeting predetermined criteria were systematically identified by an electronic medical record system. Criteria for review included patients who (1) returned to the ED within 72 hours and were admitted on their second visit, (2) were admitted from the ED to the floor and then transferred to the intensive care unit (ICU) within 24 hours, (3) expired within 24 hours of ED arrival, (4) required airway management, or (5) were referred to the QA committee as the result of complaints. Cases were randomly assigned to individual physicians not involved with the care. All cases were reviewed using a structured electronic tool that assessed the occurrence of error and adverse events. Institutional review board jurisdiction was waived by the Beth Israel Deaconess Medical Center IRB. RESULTS: During the study period, 152,214 cases were screened and 2131 cases (1.4%) met prespecified criteria for review. The incidence of error in these cases was 9.5% (95% confidence interval [CI], 8.3%-10.8%), representing an overall incidence of 0.13% among all ED patients. In cases that involved error, 50.5% occurred among patients who returned to the ED within 72 hours; 17.3% occurred among floor-to-ICU transfers; 5.4% occurred among mortality cases; 2.0% occurred among airway cases; and 24.8% occurred among cases referred as the result of complaints. The incidence of adverse events in the reviewed cohort was 8.3% (CI, 7.2%-9.6%), representing an overall incidence of 0.11% among all ED patients. In cases that involved adverse events, 48.6% occurred among patients who returned to the ED within 72 hours; 16.4% occurred among floor-to-ICU transfers; 9.0% occurred among mortality cases; 1.1% occurred among airway cases; and 24.9% occurred among cases referred as the result of complaints. CONCLUSION: Although the overall incidence of error and adverse events in EDs is low, the likelihood of such events is markedly increased among patients who return to the ED within 72 hours, among patients who require floor-to-ICU transfer within 24 hours, and among those whose cases come to attention as the result of complaints.