A comparative study of right ventricular histopathologic findings and clinical profiles in 174 cases with hypertrophic cardiomyopathy (HCM), 145 cases with dilated cardiomyopathy (DCM), and 241 cases with idiopathic arrhythmia and/or conduction disturbance with significant myocardial pathology [Electric disturbance type of cardiomyopathy (ECM)], totaling 560 cases, was made in order to clarify the role of viral myocarditis in these conditions. Postmyocarditis change (PMC) was defined by assessing serial biopsy findings in nine cases with acute myocarditis. The PMC was observed in three cases with HCM (1.7%), 22 with DCM (15%), and 21 with ECM (8.7%). The incidence of PMC was significantly higher in DCM and ECM than in HCM (P less than 0.001). The typical pattern of onset of acute myocarditis, i.e., high fever, upper respiratory infection, and gastrointestinal symptoms preceding cardiac symptoms within 10 days, was recognized in one case with HCM, eight with DCM, and 6 of 123 cases with ECM. The incidence was higher in DCM and ECM than in HCM (P less than 0.05). Of 46 cases with PMC, 12 (26%) showed this pattern, on the other hand only 3 of 377 cases (0.8%) without PMC did so (P less than 0.0001). Familial occurrence was not found in any of the 46 cases with PMC but was found in 38 of the 377 cases without PMC (P less than 0.01). This study indicates that viral myocarditis may often be the causative agent of idiopathic cardiomyopathy, especially DCM and ECM. It is also noticed that familial occurrence of the cases with PMC was never seen.