In an attempt to prove the hypothesis that virus myocarditis may be a cause of idiopathic cardiomyopathy, clinical and experimental studies were performed. Eleven patients with a presumptive or proven diagnosis of virus myocarditis were followed for one and a half to 13 years after the acute illness. One patient died in the acute stage, six recovered completely and one continued to have bifascicular block without subjective symptoms. Three patients had exertional dyspnea, cardiomegaly and an abnormal ECG three to 13 years after the onset, and two of them had an enlarged LV cavity with reduced EF and histological changes in myocardial biopsies. The clinical picture in these three cases was similar to that seen in congestive cardiomyopathy. Clinical observations of the heart in an epidemic of coxsackie B 3 virus infection among school children revealed that 49 (19%) of 263 infected children had abnormal chest-X ray, electrocardiographic or echocardiographic findings one to 10 months after the onset, however none of them developed cardiomyopathy. In experimental infection of weanling golden hamsters with coxsackie B 3 virus (Nancy strain), all animals developed acute and severe myocarditis, and the virus was detected in the myocardium. Hemodynamic data suggested decreased contractility of the left ventricle in the acute stage. Histologically the heart showed focal myocardial necrosis and cellular infiltration without calcification, findings which resemble those in human doxsackie B virus myocarditi. Thus, the golden hamster is a better animal model than the mouse in studies on virus myocarditis and postcarditic cardiomyopathy.