Abstract Idiopathic hypereosinophilic syndrome (HES) consists of a prolonged state of eosinophilia of unknown origin with organ involvement. We describe the case of a patient who developed fatal eosinophilic myocarditis. A 23-year-old woman with an 8-month history of eosinophilia presented with symptoms of myocarditis. Histological evaluation of an endomyocardial biopsy specimen revealed marked endomyocardial eosinophilic infiltration with eosinophil-rich granulomas and areas of myocyte necrosis. A terminal deoxynucleotidil transferase assay revealed apoptosis in several cardiomyocytes and vascular cells, mainly in the myocardial areas with higher eosinophil density. Evaluation of an endomyocardial biopsy specimen obtained after steroid therapy demonstrated that the eosinophils had disappeared, but there was marked myocardiosclerosis and scattered apoptotic cells. The patient slowly developed heart failure and died of sudden arrhythmic death. HES can cause severe myocarditis with extensive myocyte loss, probably due to both necrosis and apoptosis. Myocardial fibrosis may occur despite treatment, and patients may be at risk for fatal arrhythmias.