A 20-year-old woman was admitted with chief complaints of high fever and right lumbago. Abdominal computed tomography (CT) and ultrasonography revealed a right renal pelvic tumor 25 mm in diameter. Although malignancy could not be confirmed by the scan, we administered antibiotics based on a diagnosis of inflammatory pseudotumor of the renal pelvis. After treatment, no tumor was detected. In such cases,because diagnosis is generally difficult, surgery is usually performed. However, in this case, the patient showed a complete response to antibiotics.