Background: Long-term impact of interval between P wave and R wave (P-R) prolongation on prognosis of patients with successful catheter ablation of slow atrioventricular nodal pathway was investigated. Methods: Among 436 patients undergoing slow-pathway ablation for atrioventricular node re-entrant tachycardia (AVNRT), 17 (3.9%) experienced permanent P-R prolongation. Ablation target sites where conduction block was induced were located in mid- or anteroseptum. Fast junctional rhythm with ventriculoatrial conduction block was observed in eight patients immediately before atrioventricular block. Results: Antegrade slow-pathway conduction was eliminated in 16 patients, and retrograde fast- and slow-pathway conduction was abolished in all patients. There was no recurrence of AVNRT after an average of 38 ± 12 month follow-up. There was no deterioration of atrioventricular block in these patients. Average PR interval prior to hospital discharge and at the end of follow-up was 0.24 ± 0.02 sec and 0.23 ± 0.02 sec, respectively (p >0.05). Left ventricular ejection fraction remained unchanged in these patients (p >0.05). Conclusions: Radiofrequency catheter ablation of slow pathway for AVNRT is associated with a small risk of atrioventricular block. PR prolongation after successful slow-pathway ablation is associated with benign prognosis.