RESUMEN
We report a 58 year-old-man without comorbid conditions, with a history of two months of weight loss, malaise and headache. His initial laboratory analysis showed leukocytosis of 16,100/mL with 65% eosinophils and an absolute eosinophil count of 10,465/mL. Both bone marrow biopsy and aspirate showed infiltration by mature appearing eosinophils. Treatment was started with hydroxyurea, associated with prednisone without satisfactory decrease in the eosinophil count. Polymerase chain reaction showed the presence of the gene fusion product FIP1L1/PDGFRA. Imatinib therapy was initiated, resulting in a rapid and progressive reduction in the absolute eosinophil count, with normalization at the second week of treatment. The incidence of the myeloproliferative variant causing hypereosinophilic syndrome is rare. However, the dramatic response to imatinib emphasizes the need to study the presence of the fusion product FIP1L1/PDGFRA in all patients with eosinophilia of unknown etiology.