The therapy of the idiopathic hypereosinophilic syndrome (HES) is largely directed at controlling symptoms and preventing organ damage. Universally accepted therapeutic protocols for HES are still lacking. Patients are treated by different drugs and drug doses, and therapy appears to be mostly aimed at treating symptoms. However, in order to prevent organ damage the roles of the variously employed agents need to be clarified. Interferon was reported to be an effective agent in 44 patients, and our evaluation of five new cases suggested that IFN-alpha may be suitable as a first-line therapy.