A 16-year-old girl appeared with fever and dyspnea. In her peripheral blood, leukocytes were found to have increased up to 25,200/mm3, and eosinophils occupied 49% of the leukocyte population. By echocardiography, an abnormal echo layer which appeared to be thrombus was observed over the entire left ventricular endocardium including the posterior leaflet of the mitral valve, and mild mitral valve regurgitation was noted. After treatment, the abnormal echoes were markedly reduced simultaneously with the improvement of subjective symptoms.