A 9-year-old boy presented with increasing fatigue, anorexia, weight loss, fever, and absolute eosinophilia (48,000/microL). Pulmonary infiltrates occurred 3 months later. A murmur of mitral regurgitation was heard 5 months after onset of illness, and heart failure soon followed. Despite corticosteroid therapy the eosinophilia persisted intermittently until 1 month before death. The patient died within 9 months of the onset of illness. At necropsy there was cardiomegaly with subendocardial fibrosis in the right and left ventricles. Thrombi were present in the left ventricular apex and behind the posterior mitral leaflet. The findings in 12 previously reported pediatric cases are reviewed. The etiopathogenesis of the hypereosinophilic syndrome is discussed: half of the cases in children are associated with leukemia.

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