Peritoneal mesothelioma is a rare disease, especially when it arises in a cystic form with tardive and often nonspecific symptoms. While diffuse neoplasms have an unfavorable prognosis, cystic forms are usually benign. An accurate diagnosis can only be made only with electron microscopy and immunohistochemical studies. A 92-year-old woman with an ultrastructurally ascertained cystic peritoneal mesothelioma was admitted to the hospital's emergency ward, and was considered to be unusual because of the size of the mass and the patient's age. A review of the literature is made, and the clinical and diagnostic aspects of this disease are also discussed.