Peritoneal mesothelioma is an uncommon neoplasm often related to previous asbestos exposure. It is necessary to exclude other secondary peritoneal neoplasm. The application of immunohistochemical analysis in the biopsy sample is important for establishing an accurate diagnosis. We report the case of a peritoneal mesothelioma that started as a haemorrhagic ascites. After laparotomy, the initial diagnosis was peritoneal carcinomatosis from adenocarcinoma of unknown origin. The diagnosis was obtained by using immunohistochemical analysis: vimentin and keratine antibodies were positive and leu M1, antibodies were negative. The interest of our case resides in the difficulty for obtaining the diagnosis and the complicate management of refractory ascites. Our patient required intraperitoneal 5-fluorouracil for controlling the ascites.