BACKGROUND - Surgery for the treatment of atrial fibrillation may be associated with early or late postoperative atrial arrhythmias. In many cases, the arrhythmias that occur early in the postoperative course may be related to pericardial inflammation or increased catecholamine levels and tend to resolve without further therapy. In contrast, late postoperative arrhythmias often are persistent, highly symptomatic, and refractory to medical therapy. The incidence and mechanisms of these late atrial arrhythmias have not been thoroughly described. METHODS AND RESULTS - In this study, a total of 143 consecutive patients undergoing atrial fibrillation surgery from 1996 to 2005 were prospectively followed up. Sustained atrial tachycardia developed in 22 of 143 patients (15%) >8 weeks after the surgical procedure. These patients underwent electrophysiological study. A total of 25 separate tachycardias were mapped in these 22 patients, including 15 located in right atrium and 10 in the left atrium. Right atrial tachycardias included cavotricuspid isthmus-dependent atrial flutter (n=7), non-isthmus-dependent right atrial reentry (n=7), and 1 focal atrial tachycardia. Left atrial tachycardias included reentry around the mitral valve (n=3) and in the roof of the left atrium (n=7). All tachycardias were successfully treated with radiofrequency catheter ablation with no recurrences in long-term follow-up. CONCLUSIONS - Postoperative arrhythmias are not uncommon late after surgery for atrial fibrillation and typically are reentrant and related to surgically created boundaries. Radiofrequency catheter ablation is highly effective in the management of these late arrhythmias.