Background Using non-contact mapping system (NCM), we sought to determine whether detailed mapping of the activations in the left atrium (LA)-pulmonary vein (PV) junction could guide successful isolation of PVs by segmental ablation in patients with atrial fibrillation (AF). Methods and Results: In 24 patients with AF (age 51.5짹12.0, paroxysmal=18, persistent=6, total 80 PVs), PV and PV-LA conduction patterns were analyzed during atrial premature beats triggered AF (7 sec; n=412) and during pacing (n=220) at high right atrium (HRA), proximal (CSp) and distal coronary sinus (CSd), and roof of the left atrium (LA). Results: 1) PV segmental entrance patterns identified by pacing were concordant to those obtained during AF in 69%, and the conduction widths were wider during pacing (15짹6 mm) than during AF (12짹6 mm, p<0.05). 2) Wave from LA to PV propagated mostly through anterior superior circumference of left superior (LSPV, 72.9%) and right superior PVs (RSPV, 82.5%), whereas there was no preferential conduction site in left inferior (LIPV) and right inferior PV (RIPV), respectively. Electrical connection between superior and inferior PVs was observed in 71% of left PVs and 65% of right PVs. 3) Most of intra-PV conduction was circular pattern perpendicular to PV blood flow at the anterior ostia of RSPV (79%) and LSPV (72%), and posterior sides of RIPV (82.4%) and LIPV (55.6%). This circular conduction pattern was related to significant conduction delay at the LA-PV junction and promoted wavebreaks at the septopulmonary bundle (27%), lower septum (13%), and lateral mitral isthmus (4%). 4) Segmental ablation along LA-PV junction guided by mapping of these entrance patterns achieved entrance block in 76 PVs (95%), and 20 (83%) patients are AF-free with (n=7, 35%) or without (n=13, 65%) antiarrhythmic agents during follow-up of 7.1짹3.4 months. Conclusion: Detailed mapping of activation patterns through LA-PV junction during AF and pacing is highly predictable to define critical segments of electrical connection between LA and PVs, which could guide effective segmental PV isolation.