This Agency for Healthcare Research and Quality (AHRQ)–supported study evaluated the effectiveness of using designated ICD-9 codes to detect adverse drug events (ADEs). Investigators convened an expert panel to identify codes representing ADEs and then performed retrospective chart reviews with a structured tool to determine the success of this methodology. Overall, flagged ADE codes detected just more than half of ADEs causing hospital admission, making it a readily available tool but not a comprehensive one. Past studies have evaluated the use of ICD-9 codes for similar purposes and for surveillance of device-related hazards.