Background: The linear ablation of left lateral isthmus (LLI) by connecting the inferior margin of the left inferior PV ostium to the mitral annulus has been known to be effective in improving the clinical outcome and in preventing macro-reentrant atrial tachycardia following pulmonary vein isolation (PVI) in patients with non-paroxysmal atrial fibrillation (NPAF). However, bidirectional block (BDB) of LLI is difficult to achieve in some patients, and once BDB was achieved, it remains unclear whether the BDB of LLI relapses or has any incremental benefit during long term follow up. Methods: We compared the clinical outcome and redo-ablation findings in 68 patients with NPAF (58 male, 55.4짹10.4 years old) who underwent linear ablation of LLI. After circumferential antral ablation, we performed linear endocardial ablation of LLI along the vein of Marshall. If LLI block was not achievable with endocardial ablation, the ablation was performed inside the coronary sinus (CS). Results: 1. BDB of LLI was achievable in 58.0% (39/68) of patients. Among these 39 cases of complete BDB, 10.3% (4/39) could be done with endocardial ablation only and 89.7% (35/39) needed additional ablation of inside CS. 2. There was no complication related to LLI ablation in both endocardial and CS ablation group. 3. During 13.3짹6.3 months of follow-up, the early recurrence rate (30.8% vs. 42.9%, p=NS), the recurrence rate 3 month after RFCA (15.4% vs. 7.1%, p=NS), or the maintenance rate of antiarrhythmic drugs (63.9% vs. 53.8%, p=NS) were not different between the patients with successful BDB of LLI and those without BDB. 4. Atypical atrial tachycardia recurred in 17.9% of group with BDB and in 14.3% of group without BDB (p=NS). 5. Out of 6 patients with recurrences after BDB at LLI, 2 patients received redo ablation in 9.0짹4.2 months after 1st RFCA. The LLI was reconnected in both patients. Conclusion: The BDB at LLI is relatively hard to achieve with endocardial linear ablation only and needs to be done with additional CS ablation. Acute successful achievement of complete BDB at LLI dose not predict long term clinical outcome, and the reconnection of epicardial musculature may underlie this result.