Diagnostic errors are increasingly recognized as a major source of preventable patient harm. Although checklists have been shown to improve patient safety in surgery, their utility in aiding diagnostic decisions is less clear. In this study, internal medicine residents examined a high-fidelity cardiac simulator and then were given a checklist that outlined the major aspects of the cardiac exam. The checklist significantly improved residents' diagnostic accuracy. However, this benefit was seen only among residents who were allowed to reexamine the simulator after completing the checklist, suggesting that the advantage was due to careful recollection of data. An AHRQ WebM&M commentary discusses a missed physical examination finding that led to a diagnostic error.