Catheter radiofrequency ablation has changed approaches to the treatment of many types of heart rhythm disorders, especially supraventricular ones. For a number of common heart rhythm disorders, this method is not just another method of treatment, but a method of choice that allows to achieve a complete cure, as well as to save patients from the need to take antiarrhythmic drugs. Wolf–Parkinson–White syndrome is caused by the existence of an additional pathway of atrioventricular conduction, “shunting” the atrioventricular node, which leads to pre-excitation of the ventricular myocardium. The effectiveness of catheter ablation of additional atrioventricular connections is approximately 95%. Atresia of the coronary sinus is a rare congenital abnormality of the development of heart vessels and is usually accompanied by venous drainage into: the accessory superior Vena cava, the left atrium through the roof of the coronary sinus, or the heart chamber through the collateral venous pathways. Although this abnormality does not lead to intracardiac hemodynamic disorders, it creates difficulties in conducting procedures such as cardiac resynchronization therapy or catheter ablation of an additional pathway with a left-sided location, which require coronary sinus constriction. Your attention is given to a case of successful radiofrequency ablation of the left-sided additional atrioventricular junction in a patient with atresia of the mouth of the coronary sinus.