Idiopathic hypereosinophilic syndrome (HES) is characterized by marked (＞1,500/μL) and persistent (＞6 months) eosinophilia without an identifiable underlying cause. This condition is associated with multiorgan involvements. Renal involvement is rarely reported and little known about renal morphologic changes in this disease. We report a case of HES with renal involvement presenting with nocturia and dyspnea. A 30-year-old man visited to our institution due to nocturia and dyspnea. Peripheral blood tests showed an increased eosinophil count (14,868/㎣). Chest CT showed extensive areas of ground-glass opacities in both lungs. Renal biopsy showed thrombotic microangiopathic nephropathy. Based upon his medical records, physical examination findings and laboratory data, he was diagnosed as having idiopatic HES with renal involvement. We found FIP1-like-1-platelet-derived growth factor receptor-α (FIP1L1-PDGFR α) gene rearrangement on BM aspiration. He was treated with oral glucocorticoid and tyrosine kinase inhibitor (imatinib mesylate). His symptoms were improved and proteinuria disappeared. (Korean J Asthma Allergy Clin Immunol 2010;30:241-245)