Objective To comprehend the etiology and the clinical characteristics of hypereosinophilia.Methods The clinical data of 53 patients with hypereosinophilia admitted to Peking Union Medical College Hospital from November 1992 to February 2005 were analyzed.Results The causes of reactive hypereosinophilia included allergic diseases,infections,connective tissue diseases,malignant diseases,lung diseases,eosinophilic gastroenteritis.idiopathic hypereosinophilic syndrome had longer course of disease;more organ involved than reactive causes.The incidence of lymph node enlargement,spleen enlargement and thrombus was higher than the reactive.IHES had higher level of white blood cell count,absolute eosinophilic count,IgG and CRP.Conclusion The etiology and clinical characteristics of hypereosinophilia are various,so the differential diagnosis of hypereosinophilia is very important.