NCI: A rare myeloproliferative neoplasm characterized by a clonal proliferation of eosinophilic precursors resulting in persistently increased numbers of eosinophils in the blood, marrow and peripheral tissues. Since acute eosinophilic leukemia is at best exceedingly rare, the term eosinophilic leukemia is normally used as a synonym for chronic eosinophilic leukemia. In cases in which it is impossible to prove clonality and there is no increase in blast cells, the diagnosis of "idiopathic hypereosinophilic syndrome" is preferred. (WHO, 2001),NCI: A disease in which too many eosinophils (a type of white blood cell) are found in the bone marrow, blood, and other tissues. Chronic eosinophilic leukemia may stay the same for many years, or it may progress quickly to acute leukemia.