Two patients sustained multiple attacks of optic neuritis with persistent visual loss. An elevated eosinophil count was initially considered an incidental finding. Years later, the diagnosis of primary hypereosinophilic syndrome (HES) was confirmed by skin and bone marrow in one patient and by lung biopsy in the other. Treatment with hydroxyurea in one patient and with continuous low-dose prednisone in the other stopped the optic neuritis attacks, resolved systemic manifestations, and stabilized neurologic manifestations. These cases emphasize that primary HES may be a cause of recurrent optic neuritis, and that delay in diagnosis and treatment of primary HES can lead to visual morbidity.