The authors report a case of a male 86 years old patient with peritoneal pseudomixoma associated to cystoadenoma of the appendix. Preoperative CT scan showed the presence of a mass in the appendiceal site containing a gelatinous fluid collection and calcifications. Notwithstanding, diagnosis of pseudomixoma was preoperatively considered less probable due to the rarity of the affection and the advanced age. At laparatomy was revealed the presence of the large mass observed at CT scan, involving the caecum and occupying the retroperitoneal space along the ascending colon. Mucinous ascites was present without invasive mucinous implants in the peritoneum. Operation consisted in a right hemicolectomy with total removal of the retroperitoneal mass and evacuation of the mucinous ascites. Hystology confirmed the clinical diagnosis of pseudomixoma peritonei associated to cystoadenoma of the appendix. The case is of paradigmatic interest as being quite correspondent to the classical Ronnett’s definition of pseudomixoma: a clinicopathological entity characterized by mucinous ascites and non-invasive mucinous implants with a characteristic distribution and containing hystologically benign mucinous epithelium derived from an appendiceal mucinous adenoma and having a benign course. According to such definition, the authors stress the importance of an appropriate classification of the observed cases of pseudomixoma, under whose term is often collected a heterogeneous group of pathological lesions, in order to better evaluate the prognosis and treatment.