Adverse drug events are a common cause of emergency department visits in children and adults, but it is unclear how accurately emergency physicians identify medication errors as a cause of a patient's presenting symptoms. In this study, emergency department patients were evaluated by a clinical pharmacist for a possible drug-related cause of their presentation, and the pharmacists' assessment was compared to that of the treating emergency physician. Agreement was only moderate, implying that physician diagnoses may actually underestimate the true incidence of medication error–related emergency department visits.