Myocarditis due to H1N1 influenza infection has not been previously described. We report on a case of acute fulminant myocarditis caused by H1N1 influenza A virus infection that mimicked acute coronary syndrome. A 50-year-old man was admitted with dyspnea, fever, cough, vomiting, and atypical chest pain of three-day history. His body temperature, pulse rate, and blood pressure were 39.2 °C, 115 beats/min, and 80/40 mmHg, respectively. Electrocardiography showed sinus tachycardia, 1-mm ST-segment elevation, and absence of R wave progression in anterior leads, and ST depression in anterolateral leads. The chest radiogram revealed diffuse bilateral alveolar infiltrates. Cardiac enzymes were elevated. Despite treatment with aspirin, clopidogrel, low-molecular weight heparin, metoprolol, and an ACE inhibitor, he developed hemodynamic instability on the first day of admission. Echocardiographic examination showed anteroseptal, apical, and lateral wall hypokinesia, left ventricular diastolic dysfunction, and dilatation of all the chambers. There was no abnormal finding on coronary angiography. The diagnosis was considered to be myocarditis; thus, anticoagulant and antiaggregant therapies were discontinued, and empirical broad-spectrum antimicrobial treatment was initiated together with antiviral oseltamivir (2x75 mg/day). The patient's clinical condition significantly improved. Nasopharyngeal samples were positive for H1N1 influenza A virus. He was discharged on the 15th in good medical condition.