Prediction of accessory pathway location before radio-frequency ablation has become increasingly important for patients with Wolff-Parkinson-White syndrome. However, existing electrocardiographic (ECG) criteria for localization of accessory pathways have several limitations, and the polarity of delta waves has not been well defined. In the present study, 369 patients with a single anterogradely conducting accessory pathway who underwent successful radiofrequency ablation were included. The polarity of delta waves was defined and categorized in detail, and various ECG characteristics of the most preexcited QRS complexes were examined and compared with QRS complexes after successful ablation in the initial 182 patients, which included morphology and polarity of delta waves, initial 20, 40, and 60 ms segments of the preexcited QRS complex, R/S ratio in the precordial leads, R/S ratio in the frontal leads, delta wave axis in the frontal plane, polarity of delta waves in the frontal leads, and polarity of delta waves in the precordial leads. The polarity of the initial 40 ms segment of the most preexcited QRS complexes in each of the frontal leads, and the polarity of the initial 60 ms segment of the most preexcited QRS complex in each of the precordial leads proved to be the best representatives of delta wave polarity in the respective leads.(ABSTRACT TRUNCATED AT 250 WORDS)