Brenner tumors account for only 1 to 2% of ovarian tumors. Proliferating Brenner tumors are characterized by an epithelial proliferation, resembling superficial well-differentiated urothelial carcinomas. A single 29-year-old patient was operated for an ovarian tumor. On gross examination, the tumor, which originated in the left ovary, was cystic multilocular and presented polyoid excrescences. Definitive histological exam showed a papillary proliferation lined by transitional cells without stroma infiltration. Mucinous cells lined some cavities. The diagnosis of proliferating Brenner tumor was made. Clinical and pathological characteristics of this rare entity will be discussed.