Dilated cardiomyopathy, frequently caused by acute myocarditis, is a common indication for heart transplantation in pediatric patients. The prognosis of children with acute myocarditis is not well known but is believed to be poor. We report the short-term follow-up in 20 pediatric patients (mean age 22 +/- 19 months) with acute myocarditis diagnosed by endomyocardial biopsy. All patients were treated by immunosuppression (cyclosporine and steroids). Endomyocardial biopsy was repeated after 6 months in all patients and after 1 year in patients with persistent acute myocarditis. To evaluate left ventricular function, two-dimensional echocardiography was performed at the time of each endomyocardial biopsy, and left ventricular end-diastolic volume index and ejection fraction were calculated. After 6 months, endomyocardial biopsy showed persistence of acute myocarditis in 13 of 20 patients. After 1 year, endomyocardial biopsy performed in 11 of 13 patients with persistent acute myocarditis showed ongoing acute myocarditis in 10 of 11 patients. On admission to the hospital, 16 of 20 patients had left ventricular dilation (end-diastolic volume index 122 +/- 19 ml/m2; normal values 63 +/- 17 ml/m2) and 20 of 20 had decreased contractility (ejection fraction 34% +/- 11%; normal values 66.1% +/- 5.2%). After 6 months, in all patients the end-diastolic volume index decreased to 73 +/- 23 ml/m2 (p < 0.001), and the ejection fraction increased to 56% +/- 8% (p < 0.000001).(ABSTRACT TRUNCATED AT 250 WORDS)