A 66-year-old woman, who had had bronchial asthma, was admitted to our hospital because she suffered from fever, productive cough, wheezing, dyspnea, and chest pressure sensation. Her chest X-ray showed migrating infiltration and marked cardiomegaly. Her ECG at the admission revealed abnormal Q wave and T wave inversion, though that of 3 years before had been almost normal. Hematology showed leukocytosis and eosinophilia of 8,000/mm3 without abnormal cells. All immunological tests were negative and the specific cause of the eosinophilia was unknown. 2 weeks after admission, she complained of severe chest pain suddenly and her ECG showed ST elevation on V1-4 and serum CPK level was elevated to 290 IU/l. By the thrombolytic agent and anticoagulant therapy, her symptom was lightened immediately. 2 months later, we made her cardiac catheterization and myocardial biopsy. Her LVG showed a small aneurysm of the apex, though her CAG was normal finding. The biopsy revealed moderate fibrosis and cellular infiltration including a few eosinophils. We thought that eosinophilic endocarditis had existed first, and secondary embolism continued led to the small infarction. The hypereosinophilia was spontaneously normalized 2 months after admission, but the patient complained of myalgia and sensory disturbance of extremities. The biopsy of quadriceps muscle could prove neither infiltration of eosinophils nor vasculitis. But we diagnosed mononeuritis multiplex due to hypereosinophilia. Judging from various symptoms and laboratory findings, this case was included to the hypereosinophilic syndrome. We also thought allergic granulomatosis and angitis as one of the differential diagnoses, but histologically vasculitis was not proved. In this case, eosinophilia was disappeared without using corticosteroids.(ABSTRACT TRUNCATED AT 250 WORDS)