This study represents the first attempt to estimate the nationwide incidence of adverse drug events (ADEs) in ambulatory care. The investigators estimate that nearly 4.5 million ambulatory visits occur yearly due to ADEs, with older patients and patients who take more than 6 medications daily being at increased risk. The study did not assess preventability of ADEs, but prior studies have found that many patients suffer preventable ADEs in the outpatient setting; these were often linked to known high-risk medications. An AHRQ WebM&M commentary discusses an outpatient ADE related to a pharmacy dispensing error, and the Patient Safety in Ambulatory Care Primer discusses ADEs and other common outpatient safety issues.