Adverse drug events (ADEs) are common in ambulatory care. One classic study estimated the incidence of medication errors in outpatients at 27 per 100 patients over a 4-week period, higher than in hospitalized patients. However, ADEs may be difficult to identify in routine practice, as patients are not monitored as closely as in the inpatient setting. This study screened electronic medical records for evidence of ADEs using several different algorithms, and identified one ADE for every seven patient-years, most of which were not considered preventable. The highest yield screening algorithms were triggers that identified patients with abnormal lab values in combination with high-risk medications. An outpatient medication error due to a pharmacy dispensing error is discussed in an AHRQ WebM&M commentary.