Catheter ablation with the use of radiofrequency energy has been used as a treatment for atrioventricular reciprocating tachycardia in patients with Ebstein's anomaly. However, the success rate in these patients is generally low. We experienced a case in which the decision regarding precise accessory pathway (AP) localization was impaired by the presence of an abnormal electrogram in the atrialized right ventricle (ARV) in a patient with Ebstein's anomaly. After the AP conduction was abolished, the abnormal electrogram was observed behind the ventricular wave in the ARV during sinus rhythm. Furthermore, the interval between ventricular potential and the abnormal electrogram observed in the proximal ARV was longer than that in the distal ARV. We suggest that the abnormal electrogram might represent delayed local ventricular activation, and that the potential conduction might be impaired in the ARV.