A case of primary retroperitoneal mucinous cystadenoma in a 67-year-old woman is reported, associated with a microinvasive vulvar carcinoma. The cystic tumor measured about 30x20x10 cm and the ovaries appeared normal. The histogenesis of this rare tumor is uncertain: most Authors suggest that it develops through mucinous metaplasia in a pre-existing mesothelium-lined cyst.