In this study, we tried to disclose certain electrocardiogram (EGG) criteria that might be useful in the classification of posteroseptal accessory atrioventricular pathways as right and left in patients with pre-excitation in whom the accessory pathway localization was verified by subsequent successful ablation. Twenty such patients with posteroseptal accessory pathways (mean age 34.9 +/- 9.8; 11 male, 9 female) were included in the study. Localization of the accessory pathway was right posteroseptal in 13 (65%) and left posteroseptal in 7 (35%). Common to all these 20 patients with posteroseptal accessory pathways was a QRS polarity positive in lead L1 and negative in leads D3, aVL. In patients with right posteroseptal accessory pathways, QRS polarity was negative in lead V1 in all and positive in lead V2 in 90%. On the other hand, none of the patients with left posteroseptal accessory pathways showed negative QRS polarity in lead V1.