Backgrounds: In patients with an AP close to the His bundle, RFCA requires additional care to avoid damaging the normal conduction system. The purpose of the study was to investigate the electrocardiographic and electrophysiologic characteristics of anteroseptal(AS) APs. Methods: From November 1997 to July 2006, 4192 patients with AP-mediated tachyarrhythmia underwent RF catheter ablation in this institution, and 48(1.1%) patients (30 men and 18 women; mean age, 3315 years) had AS APs. The delta wave and precordial QRS transition during sinus rhythm, electrophysiologic characteristic and catheter ablation in 48 patients with AS AP were analyzed, retrospectively. Results: 48 patients with AS AP, 19 with manifested AP (18 with constant Wolff-Parkinson-White (WPW) syndrome, 1 with intermittent WPW syndrome), 23 with concealed AP ( 18 with AVRT, 5 with AT) and 6 with multiple APs, underwent attempts at ablating their pathways using radiofrequency energy. 44 APs were successfully ablated without complication during the first session. A second attempt at ablation was made in 4 patients with success (3 recurred cases, the other one failed case at the first session). Mean power administered for the successful RF ablation was 50짹19 W; mean temperature was 54짹8���; mean duration of RF ablation was 42짹9 sec. Procedure related AV block and valvular damage did not occur. In the surface 12-Lead ECG, the delta wave polarity was positive in lead II (21/21), lead III (17/21), aVF (21/21), V1 (11/21, biphasic delta wave 4/15), and V2 (14/21, biphasic delta wave 4/15) in patients with manifest AS AP. R/S was >1 in lead III in 16 of the 21 patients with manifested AS AP. Furthermore, in patients with manifested AS AP, precordial QRS transition was noted in lead V1-2 in 1 patient, lead V2-3 in 9 patients, lead V3-4 in 9 patients, and lead V4-5 in 2 patients. Conclusions: RFCA of AS AP requires utmost care because of possibility of damaging the normal conduction system. However, with comparison to RFCA of other located AP, RFCA of AS AP did not differ significantly in terms of success rate, recurrence rate, and RF power and temperature applied during the procedure.