Hypereosinophilic syndrome (HES) is a rare disease with broad clinical signs and symptoms which is diagnosed based on a persistent blood eosinophil count of greater than 1500 cells, various end-organ damages (including skin, heart, lung, nervous system and digestive system etc.), and with exclusion of known secondary causes of hypereosinophilia. HES has a high morbidity/mortality rate. The major treatment of HES has been systemic corticosteroid and other chemotherapeutic drugs (for example, hydroxyurea and interferon) with the intention to lower eosinophil counts and therefore to slow down the progression of disease. Even though corticosteroid and other therapies can effectively reduce eosinophilia in some patients, some may eventually become nonresponsive and intolerable to the amount of side effects of the long-term therapy with these medications. Mepolizumab is a humanized monoclonal antibody that binds specifically to human interleukin 5 (hIL-5) and inhibits its activity. Previous human experience has shown it has been effective in reducing blood eosinophilia in atopic and HES patients and has alleviated some HES clinical signs and symptoms. This study intends to further evaluate the corticosteroid-sparing and clinical benefit of mepolizumab in HES.