The authors conducted histological, immunohistochemical, and morphometric studies of 47 endomyocardial biopsies and 33 hearts with a high morphological criterion for the diagnosis of arrhythmogenic right ventricular dysplasia after F.I. Marcus et al. (2010), as well as intravital and postmortem cardiac magnetic resonance imaging (MRI). The average residual cardiomyocyte area was 39.7±12.8%, and the average cardiomyocyte diameter was 10.4±1.5 µm. Muscle fiber atrophy, lipomatosis, and fibrosis in the right ventricle were detected in all cases, myocarditis in 44%, mitral valve prolapse in 42%, and muscle fiber dyscomplexation in the right ventricular in 33%. The most informative signs for the diagnosis of arrhythmogenic right ventricular dysplasia using MRI are right ventricular contraction dyssynchrony and those using a morphological study are a residual cardiomyocyte area and muscle fiber atrophy in the right ventricle. A decrease in or focal absence of γ-catenin expression in the muscle fibers was observed only in 60% of the patients.