SummaryIdiopathic hypereosinophilic syndrome is characterized by prolonged eosinophiliaof undetected cause and multiple organ system involvement (lung, kidney, nervous system, skin,…). Nevertheless, the prognosis has been correlated with heart involvement, which usually results in a restrictive cardiomyopathy with apical obliteration by fibrosis, mural thrombi and mitral and tricuspid regurgitation.This disease has a wide range of severity: some patients suffer from a real myeloproliferative syndrome and may develop blastic transformation while others present only skin involvement or are asymptomatic.Corticosteroids and hydroxyurea are both effective treatments. Interferon a seems to be active for the myeloproliferative form of the disease.Cytotoxic activity of activated eosinophil granular proteins may play an important role in tissue damage. The cause of eosinophilic proliferation (primitive malignant proliferation or resulting from a T lymphocyte stimulus) and activation remains uncertain.