Eosinophilia syndromes are heterogenous disorders possessing different clinical manifestations characterized by peripheral blood eosinophilia and end organ damage. Treatment includes glucocorticoids, hydroxyurea (hydroxycarbamide), chemotherapy and interferon-alpha. Recently, an explanation for the eosinophilia has been found in many patients who would previously have been regarded as having an idiopathic hypereosinophilic syndrome. Such cases have fallen into myeloproliferative and lymphoid categories with some cases remaining unexplained. A subgroup of patients with the myeloproliferative variant carry the new gene rearrangement FIP1L1-PDGFRA, which produces a constitutively active tyrosine kinase often responsive to antityrosine kinase therapy with imatinib mesylate. A newly developed drug currently being tested in clinical trials is the humanized monoclonal antibody against human interleukin-5. Interleukin-5 positively regulates eosinophil growth, activation, survival and tissue recruitment. As such, anti-interleukin-5 therapy is expected to be very useful, especially for patients with the clonal T-cell variant that secretes high levels of interleukin-5. This review will analyze the etiology, classification and new options for the treatment of the hypereosinophilic syndromes, with particular emphasis on anti-interleukin-5 therapy.