Background: It has been demonstrated that advanced interatrial block (IAB) is associated with an increased risk of atrial fibrillation (AF); however, the impact of advanced IAB on recurrence of paroxysmal AF after catheter ablation is not clear. Methods: 204 consecutive patients with paroxysmal AF who underwent index circumferential pulmonary vein (PV) isolation were prospectively enrolled. In all patients, a resting electrocardiogram in sinus rhythm was evaluated for the presence of advanced IAB, defined as a P-wave duration >120 ms and biphasic (+/-) morphology in the inferior leads. Advanced IAB was detected in 20.1% of patients. AF recurrence was defined as the occurrence of confirmed atrial tachyarrhythmia lasting more than 30 s beyond 3 months after the catheter ablation in the absence of any antiarrhythmic treatment. Results: During the mean follow-up period of 13.9 +/- 6.2 months (range, 3-27 months), 62 patients (30.4%) developed recurrence of AF. The recurrence rate was higher in patients with advanced IAB than those without advanced IAB (46.3% vs. 26.4%, p = 0.006). Cox regression analysis with adjustment for age, P-wave duration, CHADS(2) score, and PV isolation identified advanced IAB (hazard ratio, 2.111; 95% confidence interval, 1.034-4.308; p = 0.040) and left atrial diameter (hazard ratio, 1.051; 95% confidence interval, 1.004-1.100; p = 0.034) as two independent predictors of recurrence of AF. Conclusions: Patients with advanced IAB were at an increased risk of AF recurrence after catheter ablation. (C) 2015 Japanese College of Cardiology. Published by Elsevier Ltd. All rights reserved.