IgG4-related sclerosing disease is a novel clinicopathological entity characterized by fibrosis, extensive infiltration of IgG4-positive plasma cells, and serum IgG4 elevation. This disorder includes a variety of diseases, such as autoimmune pancreatitis, retroperitoneal fibrosis, sialadenitis, thyroiditis, inflammatory abdominal aneurysm, tubulointerstitial nephritis, and inflammatory pseudotumor [World J Gastroenterol 2008;14:3948-3955]. A 71-year-old man visited our hospital with the complaint of left flank pain and gross hematuria. Computed tomography (CT) revealed left hydronephrosis and a thick retroperitoneal soft tissue mass around the ureteropelvic junction, suspicious of renal pelvic cancer. Urine cytology using a urine sample from the left renal pelvis was negative. On laboratory examination, serum levels of IgG and IgG4 were found to be elevated. The patient refused tumor biopsy. Therefore, he was treated with corticosteroid therapy on the basis of a clinical diagnosis with IgG4-related retroperitoneal fibrosis. Regression of the retroperitoneal mass as well as improvement of left hydronephrosis and decrease in serum IgG4 levels were accomplished. These effects strongly suggested that the present case was an IgG4-related retroperitoneal fibrosis. However, in this instance, since we could not completely rule out malignancies by biopsy, careful follow-up was necessary with these points in mind.