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Idiopathic hypereosinophilic syndrome (HES) is a rare disorder characterized by peripheral eosinophilia exceeding 1500/mm 3 , a chronic course, absence of secondary causes, and signs and symptoms of eosinophil-mediated tissue injury. One of the best-characterized forms of HES is the one associated with FIP1L1-PDGFRA gene rearrangement, which was recently demonstrated as responsive to treatment with the small molecule kinase inhibitor drug, imatinib mesylate. Here, we describe the case of a 51-year-old male, whose symptoms satisfied the clinical criteria for HES with cutaneous and cardiac involvement and who also presented with vasculitic brain lesions and retroperitoneal bleeding. Molecular testing, including fluorescence in situ hybridization, of bone marrow and peripheral blood showed no evidence of PDGFR rearrangements. The patient was initially treated with high-dose steroid therapy and then with hydroxyurea, but proved unresponsive to both. Upon subsequent initiation of imatinib mesilate, the patient showed a dramatic improvement in eosinophil count and progressed rapidly through clinical recovery. Long-term follow-up confirmed the efficacy of treatment with low-dose imatinib and with no need of supplemental steroid treatment, notwithstanding the absence of PDGFR rearrangement., Competing Interests: Conflict-of-interest statement: All of the authors state that they have no conflicts of interests related to this case or publication of its findings.