We report 9-year-old boy of Löffler endomyocarditis. He was a 9-year-old boy with a complaint of long lasting low grade fever. Blood examinations at admission showed a moderate leukocytosis and marked eosinophilia (20700/mm³, 68%). His ECG showed T wave inversions in lead V_{3, 4}. The echocardiogram showed mild left ventricular enlargement and mild hypertrophy in left ventricular apex. We treated him with a corticosteroid and Ca-antagonist. Although the leukocyte count and eosinophil count returned to normal levels, his cardiac involvement did not disappear. His hematological findings stayed within normal ranges, until we stopped the therapy with corticosteroid. Immidiately after stopping steroid therapy, leukocytosis and eosinophilia developed again. His echocardiogram showed a hypertrophy in left ventricular apex, which was worse than that in the first examination. The endomyocardial biopsy which we performed on his relapse revealed "fibroplastic parietal endomyocarditis with eosinophilic infiltration", which led us to make a diagnosis of Löffler endocarditis.