Information on medication errors in the ambulatory setting is still lacking, as most research has focused on adverse drug events in the hospital setting. This study sought to address this deficiency, using data submitted through a voluntary incident reporting system. Most reported errors occurred at the prescribing stage (e.g., prescribing an incorrect dose), and nearly one in four resulted in the need for additional monitoring or intervention to prevent patient harm. Prior research using this database has analyzed the types of errors reported by staff and clinicians and examined errors in the process of ordering diagnostic tests.