We described a 15-year-old girl who presented with persistent fever, bilateral flank pain, and worsening dyspnea. The peripheral blood cell count showed remarkable eosinophilia at the time of admission. Severe pleural effusion with eosinophilic Infiltrations as well as pericardial effusion were noted thereafter. Bone marrow examination disclosed markedly increased eosinophils. Bilateral ectasia of the renal pelvis was found in an ultrasonographic study of the kidneys. Spiking fever and progressive shortness of breath persisted despite treatment with empiric antibiotics for infection. Based on the clinical course and histological findings, a tentative diagnosis of idiopathic HES was made. After treatment with oral prednisolone daily (1 mg/kg/day) for one week, there was a rapid improvement in her clinical condition. She was discharged a few days later and the steroids were withdrawn gradually when she was asymptomatic. The absolute eosinophil count (AEC) was monitored during follow-up. At 3 weeks, the AEC had fallen from 8,060/mm3 to 4,792/mm3 and it further fell to 1,591/mm3 at 5 months, and to 855/mm3 at 8 months during follow-up. There is no evidence of any other organ involvement until now. The clinical manifestations, diagnosis and management of idiopathic HES in children are also reviewed.