The aims of this study were to form a new algorithm under the guidance of radiofrequency catheter ablation (RFA) to localize the accessory pathway (AP) in patients w ith the Wolff-Parkinson-White (WPW) syndrome using the surface ECG during sinus rhythm, and to test other algorithms, which have been proposed for the same purpose. Sixty-five patients with WPW syndrome, who had only one atrioventricular AP and mani fest or intermittent preexcitation on the surface ECG were included in the study. The exclusion criteria were inadequate preexcitation (QRS duration $ 100 ın s ) and an ultirnately unsuccessful RFA procedure. The AP was Iocalized to one of the eight predetermined anatoın ical zones around the mitral and tricuspid annuli. The ECGs were analyzed for QRS complex polarity, delta wave polarity and QRS complex amplitude. Cri teria that could distinguish different AP zones were identified. The criteria with highest success were used to form the algorithm. The new algorithm correctly localized the AP in 92% of the patients to one of the seven AP zones. A differentiation between right and left posteroseptal pathways could not be achieved. Three different algorithms proposed by other investigators were tested prospectively with the same ECGs. Although the reported success rates for correct localization of the AP were 87%, 92% and 93%, these algorithms showed lower success rates in our study group (72%, 74% and 62%, respectively). W e concluded that our new ECG algorithm is a useful noninvasive tool to guide the RFA procedure; but a prospective study is needed to verify its high success rate. A drop in the accuracy of our algorithm should be expected in a different group of patients, as we have observed for the other algorithms.