Medication errors contribute to significant morbidity, mortality, and costs to the health system. Little is known about the characteristics of Emergency Department (ED) medication errors. To examine the frequency, types, causes, and consequences of voluntarily reported ED medication errors in the United States. A cross-sectional study of all ED errors reported to the MEDMARX system between 2000 and 2004. MEDMARX is an anonymous, confidential, de-identified, Internet-accessible medication error-reporting program designed to allow hospitals to report, track, and share error data in a standardized format. There were 13,932 medication errors from 496 EDs analyzed. The error rate was 78 reports per 100,000 visits. Physicians were responsible for 24% of errors, nurses for 54%. Errors most commonly occurred in the administration phase (36%). The most common type of error was improper dose/quantity (18%). Leading causes were not following procedure/protocol (17%), and poor communication (11%), whereas contributing factors were distractions (7.5%), emergency situations (4.1%), and workload increase (3.4%). Computerized provider order entry caused 2.5% of errors. Harm resulted in 3% of errors. Actions taken as a result of the error included informing the staff member who committed the error (26%), enhancing communication (26%), and providing additional training (12%). Patients or family members were notified about medication errors 2.7% of the time. ED medication errors may be a result of the acute, crowded, and fast-paced nature of care. Further research is needed to identify interventions to reduce these risks and evaluate the effectiveness of these interventions. Copyright © 2011 Elsevier Inc. All rights reserved.