In the Annuals of Autopsy Records for Japan from 1958 to 1977, 377,841 autopsy cases were registered; a short summary of pathologic findings was included. Of the 377,841 cases, 409 (0.11%) were registered as idiopathic, nonspecific, interstitial, or viral myocarditis and 25 cases (0.007%) as giant-cell myocarditis. The annual incidence of the myocarditis varied at 5-year intervals and a remarkable increase was observed after 1974. An analysis of 19 cases of myocarditis and 19 of postmyocarditic cardiomegaly (PMC) showed a preponderance of males. Dilatation of ventricles was found in all cases of acute, subacute, and healing myocarditis, which histologically showed interstitial mononuclear cell infiltration, necrosis, and disarray of myocytes. Hypertrophy with ventricular dilatation, ventricular dilatation without hypertrophy, endo- or epicardial involvement, and right ventricular lipomatosis were found in 70%, 20%, 30%, and 10% of cases with chronic myocarditis and PMC, respectively. In these cases, residual inflammation, fibrosis, and hypertrophy was observed histologically.