Idiopathic hypereosinophilia (also termed hypereosinophilia of undetermined significance, i.e. HEUS) is a disorder characterized by an increase in eosinophil blood counts above 1,500/μL, as detected on at least 2 separate examinations. The disorder cannot be associated with eosinophil-based tissue damage or a primary or secondary cause of eosinophilia. That is, it is a diagnosis of exclusion and has no known cause. Over time, this disorder can resolve into a primary hypereosinphilia, typically clonal hyperesinophilia, chronic eosinphilic leukemia, or an eosinophilia associated with another hematological leukemia. The disorder may also become associated with tissue or organ damage and therefore be diagnosed as the hypereosinophilic syndrome. Idiopathic hyereosinophilia is treated by observation to detect development of the cited more serious disorders.