Initial electrophysiologic assessment and radiofrequency ablation can be abbreviated when accurate localization of the accessory pathway is available before the procedure. Several criteria have been previously proposed for localization of accessory pathways from the surface electrocardiogram. Rosenbaum classified ECG of patients with preexcitation but did not include septal connections. Gallagher based their analysis on surgical ablation identified 10 locations around the tricuspid and mitral valve and the septum. The thoracic cycle electrocardiogram identified correctly the preexcitation location and its very useful for the differential diagnosis between right on left pathway, particularly in the posterior septal region. We also chose leads III, V1 and V2 to analyze the frontal and horizontal planes of the heart activation and we could easily identify the sites of right and left preexcitation. With our algorithm, we were able to correctly identify the location of the accessory pathway in 88% of the patients, with a high success rate of the radiofrequency ablation.