Peritoneal carcinomatosis is defined as a secondary neoplastic invasion of the peritoneum. This entity may represent a significant challenge in terms of its diagnosis, its exploration and research of its origin and its treatment. This clinical history illustrates the difficulties generated by a case of peritoneal carcinomatosis and describes the importance of a multidisciplinary approach. When the primary origin is unknown before the initiation of treatment, the peritoneal carcinomatosis is a form of carcinoma of unknown primary (CUP). When this condition affects a woman, it becomes a particular entity with a better outcome, for which a specific first-line treatment should be initiated.