Curative catheter ablation has been established as an effective therapeutic option for atrial fibrillation (AF) that is resistant to pharmacologic rhythm or rate control. However, standard ablative approaches targeting the pulmonary veins (PVs) are associated with a success rate as low as 40%. In a recent study, Kumagai et al. described a new approach of catheter ablation of AF isolating the posterior left atrium including all PVs (called Box Isolation). In Kumagai's study, 46 patients with symptomatic AF underwent box isolation. At 6 months follow up, 43 of 46 patients (93%) were arrhythmia free without antiarrhythmic drugs, with a single procedure success rate of 87% (40/46). This study provided new evidence supporting the hypothesis that the posterior wall is of high importance for the maintenance of AF. The aim of the investigators study is to determine the efficacy of two different approaches of catheter ablation (Standard PV Isolation vs. Box isolation) for the treatment of chronic AF.