BACKGROUND--Radiofrequency catheter ablation of accessory arterioventricular pathways has been shown to be a highly effective treatment in patients with paroxysmal superventricular tachycardia. There is, however, considerable variability in the number of attempted ablation sites, contributing to lengthy operations in some cases. The purpose of this study was to investigate the predictive value of local electrogram characteristics for the identification of successful target sites. METHODS AND RESULTS--Local bipolar electrograms from 60 patients recorded at 568 sites of attempted ablation were analysed. There were 511 sites at which ablation was guided by antegrade mapping, with 49 successful sites and 462 failed sites including 61 at which accessory pathway conduction was blocked temporarily. In multivariate analysis, the time of local ventricular activation relative to onset of the QRS complex (QRS-V interval) (p < 0.001) and the presence of a possible accessory pathway potential (p < 0.05) were found to be independent predictors of successful outcome, whereas the atrioventricular interval, atrioventricular amplitude ratio, stability of local electrogram, and presence of continuous electrical activity were not. A QRS-V interval of < or = +10 ms identified successful sites with a sensitivity of 98%, but the positive predictive accuracy was only 11%. Even a QRS-V interval of < or = 20 ms resulted in only a 24% probability of success with a sensitivity of just 18%. Combining the QRS-V interval with the presence of a possible accessory pathway potential greatly reduced the sensitivity at all cut off values, but failed to increase positive predictive accuracy above 25%. There were no significant differences in electrogram characteristics between the successful sites and the sites at which conduction of the accessory pathway was interrupted transiently. There were 57 sites (nine successful, 48 failed) at which ablation was guided by retrograde mapping, but none of the local electrogram characteristics analysed emerged as independent predictors of successful pathway ablation. CONCLUSION--Local electrogram characteristics used to guide radiofrequency catheter ablation are all associated with a low positive predictive accuracy, which significantly contributes to duration of the operation. It is unlikely that this problem will be resolved solely by modification of the mapping techniques. It could require advances in catheter technology and power sources to enable larger tissue lesions to be delivered more reliably than is possible with the equipment in current use.