Purpose of the study: Typical Atrioventricular (AV) Node Re-entrant Tachycardia (AVNRT) occurs in patients with dual AV nodal pathway, a rapid pathway used for retrograde conduction and a slow pathway used for the anterograde conduction. In sinus rhythm the patients have generally the signs of conduction through the rapid pathway with a normal or short PR interval. The purpose of the study was to evaluate the prevalence of patients with 1st degree AV block and with AVNRT and their clinical characteristics. Methods: 1040 patients, 366 males, 665 females were admitted for typical AVNRT. They were aged from 6 to 90 years (mean age 50±19). Initial ECG and clinical data were collected. Electrophysiological study was systematic. Results: Spontaneous 1st degree AV Block (AVB) was rare and noted in 8 patients. The prevalence of the association 1st degree AV Block and AVNRT was 0.8%. AVB was suprahisian in 7 of 8 and infrahisian in one of them. Five patients complained of AVNRT at exercise. Two patients had an ischemic heart disease. Patients with AVB were significantly older (71.5±16 years) than patients without AVB (50±19) (p<0.002). AVNRT was induced in control state in 5 patients. The rate of tachycardia was slow between 130 and 160 bpm. AVNRT was induced after isoproterenol in 3 patients and the rate was higher (180 to 200 bpm). Ablation of slow pathway was performed in 7 patients. Transitory 2nd degree AVB was noted in 1 patient. AVNRT was not inducible after ablation. PR interval remained unchanged. At atrial pacing, the rate of 2nd degree AVB occurrence decreased, due to the disappearance of the conduction through the slow pathway. Two patients developed transitory well-tolerated 2nd degree AVB one day after ablation. One patient presented apparent sinus bradycardia related to a concealed conduction through AV node. One year after ablation none of the patients required pacemaker implantation and patients were free of tachycardia. Conclusions: The occurrence of AVNRT in patients with 1st degree AV block is exceptional and concerns old patients. The prevalence was 0.8%. Ablation of slow pathway might be safely performed without a need of pacemaker implantation. Transitory 2nd degree AVB can be noted the day after ablation.