Idiopathic hypereosinophilic syndrome (IHES) is a rare disorder defined by persistent blood eosinophilia, evidence of eosinophil-associated organ dysfunction and absence of secondary causes. Eosinophilic infiltration and its mediator release can cause damage to multiple organs. Although IHES can involve every organ system, bladder involvement is rarely evidenced. We recently reported a case of IHES with both bladder and gastrointestinal tract involvement. A 43-year-old woman visited Hallym University Sacred Heart Hospital complaining of urinary frequency, abdominal pain, and diarrhea for several months. Abdominal pelvic computed tomographic scan showed diffuse wall thickenings in her bladder and colon with small pelvic ascites. Laboratory investigation showed a marked peripheral eosinophilia and tissue biopsies confirmed eosinophilic infiltration in the bladder wall, esophagus, and duodenum. The patient was treated with prednisolone and her eosinophilia and symptoms have gradually improved.