A previously healthy 27-year-old woman developed an acute cardiac failure one week after onset of influenza-like respiratory infection, and died on her fourth day in hospital. Intravital differential diagnosis included myocardial infarction because of ECG changes and massive elevation of myocardial enzymes. Autopsy revealed severe myocarditis and intact coronary arteries. At microscopic examination the myocardium was heavily infiltrated with lymphocytes, and there was a marked myocytolysis. Influenza A virus was isolated from the myocardial tissue. An immunological mechanism of myocardial damage is suggested.