Myopericarditis is an uncommon manifestation, but can be reaely a lethal complication of infectious mononucleosis. We experienced a case of infectious mononucleosis complicated by myopericarditis in which the clinical pictur was confused as acute myocarddial infarction. A 25-year-old male who presented with syncope and chest pain. The diagnosis of infectious mononucleosis was comfirmed by both a positive heterophil antibody test and a high titer of Epstein-Barr virus antibody. He was found to have completely normal findings at cardiac catheterization, including coronary arteriography. Pathologic specimen from right ventricular endomyocardial biopsy demonstrated extensive lymphocytic and eosinophilic infiltration of the myocardium.