Objective: to establish the pathomorphological features of the structure and morphogenesis of the myocardium, fetal cardiac communications in neonatal infants and babies in sudden infant death syndrome (SIDS). Subjects and methods. One hundred and three babies less than 1 year of age who had died from SIDS at home in 2004-2012 were examined using macroscopic examination of their hearts, by separately weighing their cavities on an electron balance; histological examination of specimens stained with hematoxylin and eosin, as well as with picrofuxin by the van Gieson or Nissl technique; immunohistochemical examination applying neurofilament markers; alkaline cardiomyocyte dissociation test; morphometric study of cardiac chambers, by calculating the ventricular index, measuring the area of a nucleus and cytoplasm of isolated cardiomyocytes by means of a Video-Test-Morpho computer unit; and statistical data processing. Results. There was a height/heart growth imbalance manifesting as a forward cardiac mass increment, as well as discoordination of an increase in right and left ventricular masses. The increment occurred at the expense of both ventricles with the leading role of the right ventricle within the first 3 months of life and at the expense of the left ventricle within the further months of life. Microscopic examination of myocardial histological sections showed ventricular cardiomyocyte hypertrophy in the subendocardial layers, endocardial fibrosis, and myocardial stromas. In the ventricles, the area of isolated cardiomyocytes of the infants who had died from SIDS proved to be significantly lower than that in the controls, the linear parameters of cells and their nuclei changed towards a decrease in length and an increase in diameter; moreover, there was a rise in the number of nuclei per unit area. The average stromal area increased in the myocardium. Conclusion. The findings are fully consistent with the results of current clinical trials and determine the possibility of objective differential diagnosis of SIDS, the statement of which rules out any significant pathological processes in the vital organs, primarily in the heart.