Polarity reversal mapping for localization of the left free wall accessory pathway (AP) at the atrial insertion site has been shown to be effective for successful ablation, but this technique requires atrial septal puncture. We evaluated the safety, efficacy, and reproducibility of two dimensional polarity reversal mapping at the ventricular insertion site of the accessory pathway without atrial septal puncture in symptomatic patients with manifested left free wall AP. Polarity reversal mapping under the mitral annulus by transaortic approach was performed in 10 consecutive patients with conventional ablation catheter (6 French, 4 mm tip, 2 mm interelectrode distance), during sinus rhythm or atrial pacing. A low set high, bandpass filter (0.005-400Hz) was used. Radiofrequency (RF) ablation was performed at the site of ventricular electrocardiogram polarity reversal during sinus rhythm. Polarity reversal was identified in all patients at the ventricular side of the mitral annulus. Ablation was successful in all patients without complications. The procedure time was 86.0 ± 21.1 min, the fluoroscopic exposure time was 16 ± 12 min, the number of RF applications was 8 ± 6, the power level 21 ± 7 watts, and the time to initial AP block was 3.0 ± 0.9 sec. Polarity reversal mapping is a safe and efficient technique at the ventricular insertion site. This technique might be complementary to the currently-utilized activation mapping technique.