Hypereosinophilic syndrome (HES) is a myeloproliferative disorder characterized by persistent eosinophilia associated with multiple organ damage.To increase awareness of the rare but potentially life-threatening disease and to provide a brief overview of the clinical presentation, diagnosis, and management of HES.Pertinent data from the patient's clinical course was incorporated with salient articles on the topic.A case of HES is presented, followed by discussion of the clinical characteristics, diagnosis, and management. Clinical pearls and pitfalls are emphasized for the practicing allergist, clinical immunologist, and fellow-in-training.A diagnosis of HES should be considered in patients with an absolute eosinophil count of >1500 cells/μL documented on two occasions at least 1 month apart, and/or in those with pathologic confirmation of tissue hypereosinophilia. The decision to treat HES should be guided by both the clinical presentation and results of laboratory and mutational analysis.