Acute necrotizing eosinophilic myocarditis is the most severe form of eosinophilic myocarditis, or hypersensitivity myocarditis, and is characterized by rapidly progressive congestive heart failure followed by fulminant clinical deterioration that is nearly always fatal. A 55-year-old previously healthy patient with acute necrotizing eosinophilic myocarditis was diagnosed by early myocardial biopsy and successfully treated with corticosteroids. Throughout his hospitalization, the eosinophil count in the peripheral blood remained normal (56-201/mm3). Early stage corticosteroid therapy can have dramatic effects in acute necrotizing eosinophilic myocarditis and early diagnosis by endomyocardial biopsy is recommended.