A 75-year-old man visited our hospital with the complaint of lumbago due to bilateral hydronephrosis. Computed tomographic (CT) scan revealed a thick and homogeneous retroperitoneal soft-tissue mass with isodensity to the muscle, which extended from around the kidneys to the bladder. His serum IgG4 level was extremely high. He also had proptosis. He was diagnosed as having retroperitoneal fibrosis due to IgG4- related sclerosing disease, and treated with glucocorticoid and azathioprine. After 2 months, partial regression of the mass was observed.