Clinical, echocardiographic, haemodynamic and histological evaluation was performed at initial presentation and at 2, 6 and 12–24 monthly intervals in 23 patients with biopsy-proven acute myocarditis (ACM). Late (4–5 years) follow-up in survivors included radionuclide assessment of left ventricular function during exercise. Left ventricular ejection fraction (EF) was severely impaired in 81% of patients (EF 31 ± 4.4%; mean ± SEM). After 6–8 months follow-up two groups had emerged. Group 1: normal (EF 62 ± 1.9%; N = 9); group 2: impaired left ventricular function (EF 29 ± 4.7%; N = 8), P < 0.001. Group 1 patients remained clinically normal at long-term follow-up. Histological findings indicative of dilated cardiomyopathy were found in seven patients in group 2. At late follow-up, 7–9 patients who were clinically normal had abnormal EF response to exercise (rest 68 ± 2.8% /exercise 62 ± 3.4%; P < 0.02). Altogether, characteristic features of dilated cardiomyopathy developed in 12 patients (52% ), four of whom died. Assessment of left ventricular function 6–8 months following acute myocarditis predicts outcome. Late follow-up has shown progression to dilated cardiomyopathy in 50%, and most have persistent impairment of cardiac reserve.