Prior research on diagnostic errors related to inaccurate interpretation of electrocardiograms (ECGs) has generally focused on identification of acute myocardial infarction. In this cohort study, cardiologists re-reviewed ECGs to identify the rate of false-positive and false-negative diagnoses of atrial fibrillation, the most common cardiac arrhythmia. Both false-positive and false-negative diagnoses were relatively common, especially in patients with ventricular pacemakers. The authors identified several common errors in interpretation, both by human readers and the computerized interpretation. A prior study also found that the computerized ECG interpretation frequently did not identify the correct cardiac rhythm.