Seventy endomyocardial biopsies of the left and right ventricular wall taken from 23 patients during catheterization of the heart cavities have been studied. There were three groups of patients: those with hypertrophic cardiomyopathy (HCMP) (7 patients), those with dilated cardiomyopathy (11) and 5 patients with chronic myocarditis. The most characteristic features of HCMP were as follows: 1) muscle fiber hypertrophy, 2) hypertrophy of cardiomyocyte nuclei, 3) perinuclear cardiomyocyte nimbus, 4) disorganization of muscle fibers, 5) the decrease of the nucleus-cytoplasmic ration. The degree of these parameters was scored 0 to 3. Maximum total score was 15 (100%). If histological index of HCMP was higher that 60%, the diagnosis of HCMP was considered to be confirmed. The method possesses a high sensitivity (87.5%) and specificity (95%) and can be used in the diagnosis and differential diagnosis of HCMP.