The association of atrioventricular nodal re-entrant tachycardia (AVNRT) and pre-existing prolonged PR interval is unusual. Radiofrequency (RF) ablation in such patients may be associated with an increased risk of immediate and delayed AV block. The aim of our study is to assess the long-term efficacy and safety of slow pathway ablation in this population. We studied 10 patients (4 males and 6 females) with pre-existing prolonged PR interval of 68 consecutive patients with AVNRT. All had slow-fast subtype of AVNRT. The mean PR interval was 222 +/- 15 ms before RF. The patients with pre-existing prolonged PR were older (69 +/- 15 vs. 54 +/- 17, P = 0.008) and their tachycardias were slower (387 +/- 102 vs. 323 +/- 73 ms; P < 0.05). Transient complete AV block (<5 s) occurred in two patients. None had permanent complete AV block. One patient had a significant increase in PR interval (from 220 to 320 ms). The mean post-RF PR interval was 232+/-37 ms (P = n.s.). Over a mean follow-up of 39 +/- 21 months, none had a recurrence of tachycardia nor developed higher degree AV block. In conclusion, in patients with AVNRT and pre-existing prolonged PR interval, a slow pathway ablation appeared efficient and safe. From our data, no delayed AV block developed on a long follow-up. Most of the patients with periprocedural transient AV block had no evidence of dual AV node physiology, suggesting that, in this population, absence of dual AV node physiology may be associated with a higher risk of AV block during slow pathway ablation.