The hypereosinophilic syndrome, defined as persistent eosinophilia (more than 1500 cells per cubic millimeter) of at least 6 months' duration leading to end-organ damage and without evidence of a clonal or reactive cause, is a diagnosis of exclusion. This syndrome is rare and accounts for less than 1% of eosinophilic diseases. It occurs predominantly in men and if, left untreated, usually has a progressive, fatal course. In hypereosinophilic syndrome almost any organ (including the bone marrow, heart, skin, and nervous system) can be affected either by eosinophilic infiltration or by thromboembolism.
New England Journal of Medicine - Vol. 360, No. 19, May 7, 2009