We report four cases of peritoneal serous papillary adenocarcinoma (PSPC), a rare disease; all patients had ascites and high levels of serum CA125. Clinical and radiological examinations could not differentiate the disease from peritoneal metastatic tumors and mesothelioma, and histopathological analysis including immunochemistry on the specimen obtained at laparotomy or laparoscopy was necessary for the diagnosis.