This report describes three histologically documented cases of acute eosinophilic myocarditis. These three cases illustrate the different clinical and therapeutic outcomes of this disease which can range from full recovery under prolonged corticosteroid treatment to requirement for emergency heart transplantation or death due to intractable cardiac insufficiency. In absence of specific clinical or laboratory data, diagnosis must be established in vivo by endomyocardial biopsy demonstrating eosinophil-rich inflammatory infiltration and necrotic lesions. Rapid decision-making is necessary to allow early initiation of intensive corticosteroid treatment without which the most likely outcome is death. Clinicopathological and experimental evidence suggests that acute eosinophilic myocarditis is caused by the cytotoxic effects of granule components (mainly major basic protein) released by activated polynuclear eosinophils.