The idiopathic hypereosinophilic syndrome (HES) comprises a heterogeneous group of disorders with unknown pathogenesis characterized by persistent peripheral blood and bone marrow eosinophilia and eosinophil infiltrates of multiple organs, leading to severe organ dysfunction. Lymphokine-mediated T-lymphocyte control of human eosinophilic granulopoiesis is though to play a major role in the pathogenesis of HES. Treatment of this disease with cyclosporin-A (CSA) therefore appears to be sensible. We report the case of a patient with a severe HES who failed to respond adequately to glucocorticoid treatment. With additional CSA therapy, disease activity showed a favorable remission and eosinophil counts rapidly decreased to the normal range and remained normal even after reduction of the methylprednisolone dosage to 7.5 mg daily. To date his remission has continued for more than 6 months. In the meantime we have confirmed the beneficial effects of CSA in two other cases of HES. This suggests that CSA treatment is justified, at least in combination with glucocorticoids, in severe cases of HES to prevent the side effects of a long-lasting high-dose glucocorticoid therapy.