Background: Regions of rapid and multiple deflections can be identified with high dominant frequency (DF) during sinus rhythm (SR). These areas may play a role in the perpetuation of atrial fibrillation (AF) and indicate an atrial substrate abnormality. Objective: The purpose of this study was to investigate the atrial substrate properties of the high-frequency sites in patients with paroxysmal AF. Methods: Forty patients (52 ± 12 years of age) with paroxysmal AF were studied using a three-dimensional mapping system. Spectral analysis was performed on the bipolar electrograms in the left atrium (LA) during SR. Overall, 7708 electrograms were analyzed, and the DFs higher than 70 Hz were labeled as abnormal. Results: The regional distribution of the high-DF sites in the LA could be divided into two types. Type 1 includes high-DF sites existing only in the pulmonary veins (PVs; n = 19, 6.6 ± 3.4 sites/patient). Type 2 includes high-DF sites in the LA or LA plus the PVs (n = 21, 11 ± 5.6 sites/patient). In type 1, PV isolation (PVI) could eliminate the AF with negative AF inducibility testing after the PVI in 89% of patients. In type 2, additional LA substrate modification was needed in 81% of patients because sustained AF was induced after the PVI (P<.001, compared with type 1). Multivariate analysis showed that the lower mean voltage of the LA and high-frequency sites distribution both independently predicted a positive AF inducibility after the PVI (P<.05). Conclusions: Spectral analysis during SR can detect an abnormal atrial substrate. A regional distribution of the high-DF sites predicts the efficacy of the PVI.