This study aims to develop a model to determine whether a single, oral, weight-based dose of glucocorticoid (GC) can predict clinical and biologic response to GC s over the long term in subjects with hypereosinophilic syndrome (HES). Subjects with FIP1L1/PDGFRA-negative HES, who are symptomatic with eosinophil count >1500/microL and receiving less than or equal to10 mg prednisone daily, will be enrolled. A single oral dose of prednisone (1 mg/kg rounded to the nearest 5mg) will be administered. Eosinophil count and various laboratory parameters will be assessed at 4 hours and 24 hours following prednisone administration (investigators will be blinded to the results of the eosinophil counts). The subjects will then begin GC therapy at 30 mg prednisone daily followed by a standardized taper. The lowest dose of GC at which symptoms and eosinophilia are controlled will be compared to the change in eosinophil count at 4 and 24 hours post-challenge. Mechanisms and in vitro correlates of GC resistance will also be explored.