Combined retroperitoneal and mediastinal fibrosis is a rare manifestation of an idiopathic systemic sclerosing disease. This report describes a multisystem illness that clinically could best be described as polyserositis and progressive renal failure. Pathologically, it was characterized by diffuse infiltration of retroperitoneal and mediastinal tissues with plaquelike fibrofatty connective tissue encasing the kidneys, ureters, adrenal glands, and parietal pericardium. These features are diagnostic of a systemic sclerosing disease. It is important to recognize this unusual disorder to avoid confusion with other systemic connective tissue diseases such as systemic lupus erythematosus.