Mucinous adenocarcinomas of the ovary were studied immunohistochemically for cytokeratins 7 and 18, either to determine whether the ovarian tumor was primary or a metastasis or to establish the histogenetic origin of the tumor. Primary ovarian tumors were strongly positive for both cytokeratins, while ovarian metastases from colonic cancers were negative for cytokeratin 7, as were the colonic cancers. Three of four ovarian tumors complicated by pseudomyxoma peritonei were negative for cytokeratin 7, indicating appendiceal origin. Two of seven mucinous tumors associated with dermoid cysts were negative for cytokeratin 7, suggesting gastrointestinal origin.