All subjects must meet the established diagnostic criteria for hypereosinophilic syndrome: eosinophilia greater than 1,500/mm(3) on two occasions, no secondary etiology for the eosinophilia despite careful clinical evaluation, and evidence of end organ damage (histologic evidence of tissue infiltration by eosinophils and/or objective evidence of clinical pathology in any organ system that is temporally associated with eosinophilia and not clearly attributable to another cause).