To clarify the clinical and prognostic value of the ECG, an ECG review was undertaken in 45 consecutive patients with a histologic diagnosis of active myocarditis (29 men and boys and 16 women and girls; age, 36.8 +/- 15 years; idiopathic myocarditis, 39 cases). In patients (21) with symptoms of recent onset (less than or equal to 1 month) AV block and repolarization abnormalities were the prevailing ECG features at the time of admission, and a pseudoinfarction pattern (Q waves plus ST-segment elevation) frequently heralded a rapidly fatal course ("fulminant myocarditis"). Left atrial enlargement and atrial fibrillation, left ventricular hypertrophy and LBBB, which prevailed in patients who had symptoms for longer periods, corresponded to the most severe degree of left ventricular dysfunction during the initial hemodynamic and echocardiographic evaluation. The overall mortality rate after 58 +/- 24 months from the time of diagnosis was 29%. Abnormal QRS complexes and LBBB were markers of poor survival, independently of initial indexes of left and right ventricular function, both of which indicate an increased propensity for sudden cardiac death.