An unusual case of widespread peritoneal fibrosis of unknown cause ("sclerosing peritonitis") is described. The patient presented with ascites and bowel obstruction and was found to have numerous fibrous plaques involving primarily the serosa of the small bowel. The plaques were composed of bland spindle cells, ultrastructurally characterized as myofibroblasts, set within a collagenous stroma. Progressive involvement led to the patient's death 17 months after onset of symptoms. A literature review shows similar cases reported with a variety of names and a number of associated clinical conditions, none of which was present in this patient. The pathologic features seen in this case suggest that sclerosing peritonitis represents a nonspecific reaction of the peritoneal surface to a variety of insults and is characterized by florid reactive hyperplasia of submesothelial mesenchymal cells.