There is now increasing evidence that IgG4 is closely involved in idiopathic sclerosing lesions, such as sclerosing pancreatitis and sclerosing sialadenitis. In this report, we describe a case of IgG4-related retroperitoneal and mediastinal fibroses. A 52-year-old man presented with dull back pain and was found to have a continuously surrounding paraaortic mass. A biopsy specimen taken from the retroperitoneum showed a diffuse lymphoplasmacytic infiltration intermixed with fibrosis. Many IgG4-positive plasma cells were demonstrated on immunostaining. His serum IgG4 concentration was 392 mg/dL (reference range, <70). We treated this patient with a corticosteroid, which markedly diminished the paraaortic mass along with lowering of his serum IgG4 concentration. The possible involvement of IgG4 was suggested in the pathogeneses of retroperitoneal and mediastinal fibroses in this patient. IgG4 might be useful in the clinical management of retroperitoneal or mediastinal fibrosis to differentiate them from malignant tumors and predict steroid sensitivity.