Objective: To compare the clinical efficacy of the catheters with open irrigation and the catheters with a large contact area (8 mm) during the first procedure of radiofrequency ablation of the cavo-tricuspid isthmus. Materials and methods. The study involved 85 patients (57 men, 28 women) aged from 17 to 78 years who had a documented paroxysm of supraventricular tachycardia with ECG pattern of a typical atrial flutter. Exclusion criteria considered decompensated heart failure, lack of femoral venous access, a second procedure of radiofrequency ablation of the cavo-tricuspid isthmus. Patients were randomized into two groups. The basis of allocation was formed by the iplementation in the procedure of radiofrequency ablation catheters of various types: open irrigation cathetrs and catheters with a large contact area. The procedure was carried out after obtaining of a written consent from the patients, under fluoroscopic control in accordance with the recommendations of leading international experts. The study design meant a step changing of the ablation parameters and switching to the other kinds of them. The procedure was stopped after achieving the limit of the duration of the operation, which was equal to three hours. Achievement of bidirectional isthmus block during 30 minutes of the ablation (primary endpoint) and 30 minutes after the ablation, ablation time, duration of the procedure, fluoroscopy time, complications, associated with the procedure, (secondary endpoints) were taken into account. For the processing of the resulting material and statistical analysis in study groups the Statistics 7,0 (Statsoft) software package was used. Results. Bidirectional isthmus block during the first procedure of radiofrequency ablation of the cavo-tricuspid isthmus was verified in 87% of patients. The primary endpoint was achieved in 55 patients (65%): 73% of the patients in the first group and 58% - in the second (p = 0.002). A low frequency of intraoperative complications, comparable in both groups was found. Conclusion / findings. The use of the irrigated catheters for the first procedure of radiofrequency ablation of cavo-tricuspid isthmus allowed to obtain criteria of the procedures efficiency on a large number of patients. In the studied groups there were no significant differences in the duration of the procedure, fluoroscopy time or the frequency of the complications associated with the surgery.