One hundred eighty-six consecutive patients underwent radiofrequency ablation and were divided into 2 groups: group 1 included 19 patients (13 women, mean age 50 +/- 15 years) with transient atrioventricular (AV) block during the procedure. The duration of AV block ranged from 4 seconds to 30 minutes (mean 2.8 +/- 7.0 minutes); and group 2 included 167 patients (142 women, mean age 40 +/- 17 years) without transient AV block. Follow-up was 8.6 +/- 8.3 months in group 1 and 10.1 +/- 9.4 months in group 2. No significant differences were observed between the 2 groups concerning the ablation approach (fast or slow pathway), the number of radiofrequency applications, and recurrences of tachycardia. Four patients from group 1 who underwent fast pathway ablation developed late complete AV block, whereas no patient in group 2 had such a complication (p = 0.0001). Late complete AV block occurred 20 hours, 6 days, 1 month, and 25 days after ablation, respectively, and was not related to the duration of transient AV block. Another patient from group 1 developed an asymptomatic 2:1 AV block during exercise, 3 months after slow pathway ablation. Transient AV block, a common finding occurring as often during fast as during slow pathway ablation, did not preclude recurrences of tachycardia but was associated with late complete AV block.