The idiopathic hypereosinophilic syndrome consists of peripheral blood eosinophilia of $1500/mm^3$ or more without a known cause, plus signs and symptoms of organ eosinophilia. The prognosis of HES without treatment is poor. However, about one third of the patients with this syndrome may respond to corticosteroid thrapy. Morever, the majority of the remainder may have a favorable response to hydroxyurea. We present here a case of hypereosinophilic syndrome without any identifiable causes, involving bone marrow, liver, lungs and cervical lymph node. We tried corticosteroid as a treatment but it showed no response. However the hydroxyurea showed good response.