The incidence of viral myocarditis was studied prospectively at the Central Military Hospital 1 in Finland in connection with an A1 virus influenza epidemic in Jan. 1978. Of 104 conscripts taken consecutively to hospital because of a sudden respiratory infection, 41 had serologically confirmed influenza A, 37 were serologically negative, while in 26 it was not possible to carry out complete virological analyses. Six of the serologically confirmed influenza patients had acute myocarditis on the basis of serial electrocardiographic ST segment and/or T wave changes, unresponsive to beta-blockade. The incidence of the influenza A myocarditis was thus 9% of the 67 verified and suspected cases of influenza taken together. Multidirectional echocardiography revealed regional myocardial dysfunction in all the influenza patients with myocarditis; the MB-CK isoenzyme was elevated in 3 of them. The ECG changes found in connection with influenza thus commonly indicate the presence of myocardial involvement, usually a mild one.