Although computerized provider order entry (CPOE) has been shown to prevent some medication errors, implementing these systems may introduce new errors. This observational study analyzed voided clinician orders to determine if they represented true medication ordering errors identified by the canceling provider. Investigators interviewed ordering and voiding providers and reviewed patients' medical records. They found that the positive predictive value of canceled orders representing true medication ordering errors was 93.1%. The reasons for order cancellation were more accurately reported during provider interviews rather than in the reasons reported within the CPOE system. Common ordering errors included duplicate orders and incorrectly composed orders. A past PSNet perspective discussed the impact of CPOE on patient safety.