The diagnosis of acute mild myocarditis in vaguely defined. Therefore we studied 185 consecutive young men in military service with electrocardiographic changes arousing a suspicion of myocarditis in connection with an acute infectious disease. It was possible to classify 160 patients into seven electrocardiographic groups; definite or probable myocarditis was observed in 104 patients. The electrocardiographic patterns considered characteristic for acute myocarditis were: ST segment elevations followed by T wave inversions; gradually changing T wave inversions not corrected by beta blockade; and ventricular extrasystoles more than 10 per minute triggered by acute infection. Thirty-nine subjects without myocarditis had "functional" T wave abnormalities completely normalised by beta blockade, or stable T wave inversion. The leading symptoms in acute myocarditis were fatigue and chest pains; loud S3 gallop, paradoxical cardiac pulsation, pericardial friction rub, or enlargement of the heart were noted altogether in 50% of the patients. Echocardiography disclosed segmental wall motion abnormalities related to the T wave inversions. Serum creatine kinase MB fraction increased in 70% of the acute myopericarditis patients during the ST segment elevation stage. In the non-myocarditis groups the clinical and pertinent laboratory findings remained normal. Thus, we noted in clinically mild acute infectious myocarditis clear-cut and early signs of myocardial dysfunction, suggesting that the direct and often local viral invasion of the myocardium is the basic pathogenetic mechanism. The present electrocardiographic classification based on serial tracings and beta blockade proved useful in the evaluation of patients suspected of having mild acute myocarditis.