Postmortem morphological examination of 102 cases with diphtheria who had died on the disease day 1-120 was performed. All the dead patients were adults. Dystrophic-necrotic processes in cardiac conduction and intramural nervous systems, in contractile myocardium developing on the disease day 1-8 were found. The severity and extent of these processes depended on toxic and hypoxic factors resultant from obturation or ventilation respiratory failure. Myocarditis progression undergoes 2 stages: early, exudative (beginning on the disease day 3) and late, productive (beginning on the disease day 9). The disease eventuated in myocardiosclerosis starting after 28-day course. Morphologically, diphtheritic cardiac lesion differed from cardiac damage caused by other acute infections.