Catheter ablation of the AV nodal slow pathway using radiofrequency (RF) energy has been established as the first-line curative therapeutic modality of recurrent symptomatic AV nodal reentrant tachycardia (AVNRT). In contrast to catheter ablation of the AV bypass tract, there was no useful marker to localize succesful site of the pathway. This study was performed to determine predictors of successful catheter ablation of the AV nodal slow pathway in patients with AVNRT. Methods : Forty patients (18 men, 22 women; 47.9±13.3 years) with AVNRT undergoing successful catheter ablation of the AV nodal slow pathway were included in this study, in which 217 attempts were tried to ablate the AV nodal slow pathway. Characteristics of local atrial electrogram, anatomical site at each attempt, junctional rhythm during RF delivery were analyzed (40 successful, 177 failed). Maximum difference and duration of atrial electrograms were measured and local atrial electrograms were classified into 5 types (A1, A2, B1, B2 and C type) according to the type and the degree of fragmentation. Finally, the occurrence of junctional rhythm during RF discharge and its onset time were compared between successful and failed attempts. Results : There was no significant difference in the maximum difference of amplitude and duration of atrial electrograms between successful and failed attempts. The success rate in each type of atrial electrogram was significantly different. And, the success rate in non-C type atrial electrograms (A1, A2, B1, and B2) was significantly higher than that in type C atrial electrograms (25.0% vs 10.3%, p<0.01). No significant difference was noted in success rates according to attempted sites. Junctional rhythms during radiofrequency application occured significantly more frequent in successful attempts than in failed attempts (87.5% vs 47.5%, p<0.001). The time to onset of junctional rhythm was not different between successful and failed attempts (5.2±4.9 sec vs 6.1±5.5 sec). Conclusion : Fragmented local atrial electrogram and junctional rhythm during RF energy delivery may be used to predict successful catheter ablation of AVNRT. It is recommended that RF energy should be applied to the site where fragmented atrial electrogram is recorded and terminated if junctional rhythm does not develop within 15 seconds after starting RF energy delivery. (Korean. J. Med 57:867-874, 1999)