A case of a primary retroperitoneal mucinous cystadenocarcinoma of ovarian type found at laparotomy in the presence of two normal ovaries is described. Careful examination of the tumor showed no evidence of ovarian tissue in the wall. The appearance of this tumor could be traced from areas of benign mucinous cystadenoma, through a mucinous tumor of borderline malignancy to a highly malignant anaplastic tumor containing only occasional mucicarmine positive cells. In areas, the connective tissue of the neoplasm, resembled ovarian stroma in appearance. At autopsy, the retroperitoneum was free of neoplasm, but widespread metastases, consisting of poorly differentiated adenocarcinoma were observed. This neoplasm may have arisen from displaced coelomic epithelium or from a supernumerary ovary.