Hematological disorders are the third cause of hypereosinophilia, after allergic and parasitic diseases. Hematological disorders associated with hypereosinophilias can be classified as clonal, reactive or idiopathic, and recently the improvements of cytogenetic, molecular biology and immunology have allowed to revisit numerous cases previously diagnosed as idiopathic hypereosinophilic syndrome. Reactive eosinophilias are mainly associated with lymphoma or abnormal, often clonal T lymphoid population. Clonal eosinophilia is related either to various myeloid malignancies or to a genuine myeloproliferative disorder from the eosinophile lineage, the so-called chronic eosinophilic leukaemia. Chronic eosinophilic leukaemia can be associated with recurrent genes rearrangements involving PDGFRA, PDGFRB and FGFR1 or with clonal abnormalities not yet categorized. Idiopathic hypereosinophilic syndrome remains an exclusive diagnosis in presence of moderate or severe unexplained eosinophilia with target organ damage. The purpose of the diagnostic work-up of hypereosinophilic syndrome is to evidence either an abnormal T cell population or a clonal haematopoiesis. Imatinib mesylate dramatically improves chronic eosinophilic leukaemias associated with PDGFR abnormalities, while corticosteroids are still the main treatment for the other patients. In a near future, advances could arise from identification of new genes involved in clonal eosinophilia or in alternative therapy such as the anti-IL-5 antibodies.