The histologic distinction between adenocarcinoma primary to the ovary and metastatic to the ovary can be difficult. In an effort to facilitate this distinction, we have evaluated the use of immunohistochemical techniques with antibodies to cytokeratins 7 and 20, along with antibodies to carcinoembryonic antigen. We studied 24 primary ovarian adenocarcinomas, 11 colonic adenocarcinomas metastatic to the ovary, and 10 primary adenocarcinomas of the colon. Four ovarian adenocarcinomas metastatic to the colon were also studied. All but one of the primary and metastatic colonic carcinomas were negative for cytokeratin 7, whereas all the primary and metastatic ovarian carcinomas were positive for cytokeratin 7. The tumors metastatic to the ovary were all positive for cytokeratin 20 and carcinoembryonic antigen. Among the primary ovarian carcinomas, none of six serous tumors, three of seven endometrioid tumors, and three of 11 mucinous tumors were positive for cytokeratin 20. Ten of the primary ovarian tumors were negative for carcinoembryonic antigen using both monoclonal and polyclonal antibodies. One of the ovarian tumors was negative for carcinoembryonic antigen with the monoclonal antibody but positive using the polyclonal antibody. Cytokeratin 7 is the most helpful marker in the distinction between primary ovarian carcinoma and colonic carcinoma metastatic to the ovary.