Local endocardial electrograms recorded at successful radiofrequency ablation sites during anterograde mapping (QRS-V interval, atrioventricular [AV] interval, AV ratio, the presence of accessory pathway potential, and stability of atrial and ventricular electrogram) were analyzed and compared according to location of accessory pathways. The mitral and tricuspid annuli were divided into 10 regions. Endocardial electrograms differed with regard to location of accessory AV pathways. The QRS-V interval was more negative in right posteroseptal, right free wall and right anteroseptal locations than the interval in other locations. The AV interval was longer in posteroseptal than the interval in left and right free wall sites. The stability of atrial and ventricular electrograms was better in left free wall sites than in posteroseptal sites and worst in right free wall and right anteroseptal sites. The variation in local electrograms at successful ablation sites with respect to pathway location may partly explain the low predictive value and the marked variation in previously suggested criteria for selecting target sites for radiofrequency energy delivery. The results also imply that the influence of accessory pathway location must be considered when attempts are made to establish electrogram-based criteria for predicting successful ablation of accessory pathways.