The material of this retrospective study consist patients without preoperative history of atrial fibrillation, participated in three different randomized controlled trials previously . A total of 519 consecutive patients underwent coronary artery bypass surgery, aortic valve replacement or combined aortic valve with coronary bypass surgery in Kuopio University Hospital from 2004 to 2008. Patients with previous episodes of atrial fibrillation (AF) or flutter and also patients with mitral valve surgery were excluded. All patients had continuous postoperative ECG monitoring for at least 48 hours. AF episodes lasting longer than 5 minutes were registered. Thereafter 12 lead ECG was recorded daily and in case of symptoms of rhythm disturbances.