The article presents a clinical case of development of mural infective endocarditis (IE) with formation of vegetations in the region of the upper third of the interventricular septum in patient A., 51 years old, with uncorrected tetralogy of Fallot. The patient was admitted with complaints of increased heart rate, interruptions in the heart work, feet swelling, shins up to the upper third, dyspnea at rest, general weakness, fever up to 39 оC, sweating, headache. During transthoracic echocardiography on the upper third of the interventricular septum (IVS) from the side of the RV, an additional echo-positive linear mobile formation 0.9 cm long is visualized, similar to the chords of the tricuspid valve. Thus, based on the identification of 1 major Duke criterion (imaging criterion — the presence of parietal vegetation) and 3 minor criteria (predisposing factors — congenital heart disease of the blue type; fever >38 оC; positive blood culture not meeting the requirements for the major criterion), laboratory and instrumental diagnostic data and anamnesis, the following diagnosis was established: "Secondary infective endocarditis with damage to the upper third of the interventricular septum". Taking into account the sensitivity of the identified microflora, polymyxin and moxifloxacin were prescribed. During therapy, a dry cough and dry wheezing in the projection of the middle lobe on the right appeared. To exclude septic embologenic pneumonia, computed tomography of the chest organs was performed, on which no focal/infiltrative changes were detected. However, a shunt connecting the brachiocephalic trunk with the right pulmonary artery was visualized, which may explain the cause of compensation of hypoxemia that arose due to pathological hemodynamics as a result of high pulmonary stenosis. Given the incomplete effectiveness of therapy, as well as the sensitivity of the infectious agents isolated, a decision was made to change antibacterial therapy to vancomycin in combination with imipenem. Conservative treatment methods allowed not only to compensate for the existing clinical symptoms, but also to achieve reference values of laboratory parameters, despite the impossibility of surgical intervention.