In order to clarify the prognosis of myocarditis and the relationship between myocarditis and idiopathic cardiomyopathy, 20 patients with myocarditis (one with Coxsackie B; one with rubella and 18 with idiopathic myocarditis) were followed up for a long period using echocardiography and Holter electrocardiographic monitoring. The follow-up period was 49.1 +/- 39.3 months (mean +/- SD). Subjects were classified into the following 4 groups according to their prognoses, left ventricular end-diastolic dimensions (LVDd) and the presence of absence of life-threatening ventricular arrhythmias: Group I with a fatal prognosis, Group II with LVDd greater than or equal to 55 mm, Group III with LVDd less than 55 mm but associated with life-threatening ventricular arrhythmias, and Group IV with LVDd less than 55 mm and with no life-threatening ventricular arrhythmias. Patients of Group I (2 cases) had a marked left ventricular dilatation and a poor left ventricular function just before death. Patients of Group II (5 cases) had left ventricular and left atrial dilatation, and 2 of them had serious ventricular arrhythmias. All 3 patients of Group III had ventricular arrhythmia (ventricular tachycardias, coupled premature ventricular contractions and multifocal premature ventricular contractions, respectively), and 2 of them had asymmetric septal hypertrophy. All 10 patients of Group IV had no residual cardiac abnormalities. In conclusion, 50% of 20 myocarditis patients had residual cardiac abnormalities; 6 patients (2 of Group I and 4 of Group II) were complicated by left ventricular dilatation, simulating dilated cardiomyopathy, and 3 (one of Group II and 2 of Group III) showed asymmetric septal hypertrophy, simulating hypertrophic cardiomyopathy.