In an attempt to prove that myocarditis may be a cause of idiopathic cardiomyopathy, 10 cases with acute myocarditis were involved in a long-term follow-up study. There were 9 males and 1 female patient, ranging in age from 22 to 63 years. The etiology of myocarditis was idiopathic in 9 cases and rubella virus in 1 case. Clinical findings in the acute stage consisted of congestive heart failure in 6 cases, Adams-Stokes syndrome in 2 cases and cardiomegaly in 2 cases. The mean follow-up period was 55 months. Follow-up studies included physical examinations, 12-lead ECG, chest X-rays and two-dimensional echocardiograms. Various patterns of residual ECG abnormalities in the chronic stage were found, such as conduction disturbance in 1 case, pseudoinfarction pattern in 4 cases, ST-T changes in 2 cases and premature ventricular contractions in 2 cases. The cardiothoracic ratio of all cases was 60 +/- 4% in the acute stage. Two cases (Case 1 and 2) died 16 and 36 months after the acute onset, respectively. Four cases had residual cardiomegaly even in the last study period. An echocardiographic follow-up study of 7 cases disclosed progressive left ventricular (LV) dilatation and dysfunction in 3 cases, regression of LV dilatation in 2 cases and stable LV function in 2 cases. Two cases out of 3 with progressive LV dilatation and dysfunction expired after the acute illness. It was therefore suggested that acute myocarditis may cause LV dilatation and/or wall hypertrophy and that idiopathic cardiomyopathy may represent the end-stages of previous myocarditis.