Two patients with idiopathic hypereosinophilic syndrome (HES) refractory to treatment with corticosteroids and hydroxyurea received alpha-interferon (alpha-IFN) for 3 and 1 years, respectively. Eosinophil counts dropped below 1.5 x 10(9)/l at weekly doses of 2-7 x 3 million units alpha-IFN. More than a year after discontinuation of alpha-IFN, 1 patient remains in a stable remission. Granulocyte-macrophage colony-stimulating factor (GM-CSF) plasma levels were found to be normal in this patient, although GM-CSF is known to stimulate eosinophil proliferation in vitro. Based on the favorable clinical results of alpha-IFN, further studies are necessary to define its role in the treatment of HES.