Objective — to analyze cardiomyocyte (CMC) ultrastructural changes in the right ventricle outflow tract (RVOT) of infants with tetralogy of Fallot (TF) in the first years of life and to compare the findings with clinical parameters in these patients. Material and methods. Intraoperative RVOT myocardial biopsy specimens obtained from 51 patients aged 3—33 months with TF during radical correction of defect were investigated. CMC diameter and length were measured using the semithin myocardial sections stained with periodic acid-Schiff. The ultrathin sections were examined in the electron microscope. Results. The diameter of CMCs in the RVOT of infants with TF varied significantly (7.3—17.0 µm) and averaged 10.8±2.2 µm; a large number of multinucleated CMCs were observed. There were ultrastructural signs of incomplete differentiation of CMCs: active myofibril assembly in the free sarcoplasmic region; gap junctions on the lateral surfaces of CMCs; and centrioli in their sarcoplasm. Myofibrillogenesis in babies under 6 months increased in response to hemodynamic overload and hypoxemia. In addition, organelles suggestive of the synthetic activity of CMCs, such as cisterns and vesicles of the Golgi complex and granular endoplasmic reticulum, were detected in the sarcoplasm of a number of CMCs. TF infants’ myocardium also displayed focal disorders of CMC interposition; the change in the shape of myocytes was accompanied by the appearance of additional lateral insert discs. Some CMCs showed the abnormal localization of the nucleus beneath the sarcolemma, sarcoplasmic bulging areas, and dystrophic changes. Conclusion. There were ultrastructural features characteristic for the myocardium that was at the state of active growth and differentiation (increases in the diameter and length of CMCs and in the number of nuclei; myofibrillogenesis; signs of synthetic and proliferative activity along with insignificant dystrophic changes) in the CMCs of myocardial RVOT in infants with TF in the first years of life.