Diagnostic errors are increasingly recognized as a major source of preventable patient harm. Researchers compiled several studies to estimate the frequency of these errors in outpatient care across the United States. Two studies used a trigger approach to review unusual patterns of return visits in primary care and one study included consecutive lung cancer cases. From these data, the authors determined that approximately 5% of adults in the US, or more than 12 million individuals, experience a diagnostic error in the outpatient setting every year. This is the first study to evaluate the frequency of ambulatory diagnostic errors, and the results underscore the importance of efforts to improve diagnosis by addressing cognitive and systems vulnerabilities. A recent AHRQ WebM&M commentary describes a delayed diagnosis in outpatient care.