Diagnostic errors are one of the most common types of preventable errors in ambulatory care, according to data from closed malpractice claims. Difficulty in identifying missed and delayed diagnoses has hampered progress in addressing diagnostic errors. In this case-control study, investigators assessed two triggers for identifying possible cases of diagnostic error within an electronic health record. These triggers were refined from a prior study by the same investigators. The trigger methodology was reasonably accurate in identifying likely diagnostic errors, although the study was limited by poor interrater reliability between physician reviewers on whether an error occurred. Nevertheless, this study demonstrates the potential of screening approaches within electronic medical records for identifying and categorizing possible diagnostic errors.