Patients with idiopathic hypereosinophilic syndrome (HES) show persistent hypereosinophilia of unknown etiology that is associated with end-organ damage. Different treatments, including the use of corticosteroids and cytotoxics, have been investigated for HES with modest success. We describe a patient with HES who had significant end-organ damage from hypereosinophilia and remained refractory to conventional therapy. Therapy with imatinib mesylate, a selective tyrosine kinase inhibitor that is highly effective in treating patients with BCR-ABL-positive chronic myeloid leukemia, was tried with the patient. The result was impressive, with hematologic remission achieved after 12 days of administration. Our finding concurs with recent reports that imatinib mesylate may be a promising agent in the treatment of some cases of HES.