Hypereosinophilic syndomes (HESs) include potentially lethal multisystem disorders characterized by eosinophilic infiltration of a variable spectrum of target organs, predominantly the skin, heart, lungs, gastrointestinal tract, and nervous system. Based on recent advances in molecular and genetic diagnostic techniques and increasing experience with differences in clinical features and prognosis, subtypes have been defined, including "myeloproliferative-HES", "lymphocytic-HES", "familial eosinophilia", "overlap HES", "undefined HES" ("complex undefined HES", "simple undefined HES", "episodic undefined HES") and "eosinophil associated diseases" (such as Churg-Strauss syndrome). HES should be kept in mind in the differential diagnosis of eosinophilic lung diseases especially in patients with peripheral eosinophilia and pulmonary infiltrates. Corticosteroids represent an effective firstline approach to decreasing eosinophil counts in the majority of cases. Imatinib might be used for corticosteroid nonresponders. We herein report a patient with "complex undefined HES" who had disease resistant to corticosteroids, but who had a significant response after treatment with imatinib.