Alexander J Sweidan,1,2 Adam K Brys,3 David D Sohn,1,2 Milan R Sheth4 1University of California Los Angeles, Los Angeles, CA, USA; 2Department of Internal Medicine, St Mary Medical Center, Long Beach, CA, USA; 3School of Medicine, Duke University, Durham, NC, USA; 4Long Beach Memorial Hospital, Long Beach, CA, USA Abstract: Hypereosinophilic syndrome (HES) encompasses numerous diverse conditions resulting in peripheral hypereosinophilia that cannot be explained by hypersensitivity, infection, or atopy and that is not associated with known systemic diseases with specific organ involvement. HES is often attributed to neoplastic or reactive causes, such as chronic eosinophilic leukemia, although a majority of cases remains unexplained and are considered idiopathic. Here, we review the current diagnosis and management of HES and present a unique case of profound hypereosinophilia associated with warm autoimmune hemolytic anemia requiring intensive management. This case clearly illustrates the limitations of current knowledge with respect to hypereosinophilia syndrome as well as the challenges associated with its classification and management. Keywords: hypereosinophilia, eosinophils, myeloproliferative disorder, autoimmune hemolytic anemia, idiopathic autoimmune hemolytic anemia, leukemia