The use of trigger methodology to detect medical errors, including adverse drug events, has been widely validated in adult inpatients. This study reports on the development of a trigger tool to detect medication errors in pediatric patients. Using techniques similar to prior studies in adults, researchers developed a list of clinical events that may have signified an adverse event, such as use of vitamin K to treat coagulopathy. Trained reviewers retrospectively reviewed all charts where a trigger was found for evidence of an adverse event. The trigger tool identified a much higher rate of adverse drug events than found previously in pediatric patients and proved superior to voluntary error reporting at identifying both preventable and nonpreventable errors.