A 73-year-old man presented a 20-year history of recurrent, progressive, cystic lesions of the peritoneal surfaces which had necessitated two previous operations. Diffuse involvement of the peritoneum was observed at his most recent laparotomy and an attempt was made to excise as much mesothelial surface as possible. Benign cystic mesothelioma was diagnosed, based on the clinical behavior of the lesion and the presence of specific histologic and ultrastructural findings. While this lesion shares some histologic similarities with previously described peritoneal cysts, the ultrastructural demonstration of its mesothelial origin and its recurrent progressive behavior are without precedent.