Atrial fibrillation (AF) is mainly triggered by arrhythmogenic foci originating from atrial myocardial extensions (MEs) into the pulmonary veins (PVs). Aim of the study was to evaluate endocardial voltage maps of PVs as a surrogate parameter for the extent of MEs in subjects with AF through a ultra-high-density mapping system. Sixty-four bipolar electrograms were recorded simultaneously from the Orion mini-basket catheter placed in 72 PVs of 18 consecutive patients with paroxysmal/persistent AF undergoing PV isolation (PVI). The Rhythmia system in conjunction with the Orion mini-basket catheter was utilized to create a bipolar electro-anatomic reconstruction of the left atrium and PVs. Mean patients' age was 61 +/- 10 years, 56% had paroxysmal and 44% persistent AF. Mean endocardial bipolar voltages in the PVs were as follows: 1.06 +/- 0.34 mV in right superior PV, 1.14 +/- 0.52 mV in right inferior PV, 1.15 +/- 0.44 mV in left superior PV and 0.94 +/- 0.44 in left inferior PV. PVs had no detectable endocardial signals in 7 out of 72 PVs (9%); a total of 29/72 PVs (40%) revealed a non-uniform arrangement of MEs. The area of MEs was significantly larger in the superior PVs compared to the inferior PVs (9.3 +/- 4.7 vs 6.7 +/- 4 cm(2), p = 0.002). No statistical differences in terms of MEs length were found among PVs and according to type of AF. In this pilot study using a ultra-high-resolution electro-anatomical mapping system, endocardial voltage maps of PVs as a surrogate parameter of MEs among patients with AF well correspond to previous data from histopathological studies.