Checklists have been shown to improve patient safety and clinical outcomes when used in combination with other interventions to increase adherence to proven safety strategies. The use of checklists to prevent diagnostic errors has been advocated as well. In this study, medical students, residents, and cardiology fellows were instructed to use a checklist while interpreting electrocardiograms (ECGs). Use of the checklist increased the accuracy of ECG interpretation for all groups and was most effective with the least experienced clinicians. As ECG interpretation errors are common and have serious clinical consequences, checklists such as this have the potential to improve patient care.