We assessed preoperative cardiac physiology using echocardiography in patients undergoing cardiac surgery to identify predictors of postoperative atrial fibrillation. Subjects with enlarged left atrial volume had a 5-fold greater risk of postoperative atrial fibrillation, independent of age and other risk factors, than those without enlarged left atrial volume. Left atrial volume appears to be a powerful tool to stratify patients according to risk before surgery and to effectively target preventive therapy.