Considering the high incidence of infectious mononucleosis, the clinical recognition with pathologic confirmation of myocarditis due to this disease is rare. This complication has been known for many years, having been described by British authors,1 who noted cardiac dilatation and transient murmurs occurring in cases of "epidemic cervical adenitis." Reports of electrocardiographic changes date back to Longscope,2 who noted inverted T waves. With more routine use of the electrocardiograph the incidence of transient abnormalities, such as T-wave changes and conduction disturbances, has been found to be quite high, occurring in 41% of one series.3 In a recent editorial evaluation of the subject, Houck 4 reemphasized the need for cautious interpretation of T-wave changes, stating that they may occur in the presence of fever, drugs, and autonomic nervous system disturbances. A review of the literature reveals a paucity of pathologic material available for the study of myocardial involvement