Introduction: In patients with paroxysmal atrial fibrillation radiofrequency ablation of the pulmonary veins has shown superiority compared with antiarrhythmic medication. In non-paroxysmal atrial however, atrial remodeling caused by extensive fibrosis in the left atrium involves additional substrate ablation. Our study aims to measure the degree of fibrosis detected by voltage criteria in non-paroxysmal AF patients. Methods: Between November 2010 and June 2014 we performed 46 procedures of ablation for paroxysmal and non-paroxysmal atrial fibrillation. We selected only those procedures with voltage map having suffi – cient number of points representative for the entire left atrium voltage. The map was carried out during atrial fibrillation. Voltage references were: <0.,2 mV = area of scar (fibrosis), >0.5 mV = normal voltage; between 0.2-0.5 mV = borderline area. The following measurements were made: the area of the zone of fibrosis; the total area of the left atrium, the ratio between area of fibrosis and total area of the left atrium, the perimeter of fibrosis. Results: Of the 46 ablation procedures 12 non-paroxysmal AF were performed. Seven patients had sufficient points of voltage map (average 210 points) to be representative of the entire left atrium voltage. The mean area of fibrosis was 21.3 cm², mean left atrial area: 245 cm², the ratio of the total area of fibrosis to left atrium =8.7% mean fibrosis perimeter = 18 cm. Conclusions: In our cohort of patients with AF fibrosis area averaged 8.7%. Th is percentage would enroll in Class II in UTAH classification for fibrosis quantification by MRI, a percentage that includes patients in the radiofrequency ablation indication.