Cytokeratin 7 (CK-7) is a simple epithelial keratin that may be used to investigate the site of origin of adenocarcinomas. In fact, CK-7 is present in ovarian epithelial neoplasms but is generally absent in colonic carcinomas. This pattern of CK-7 expression may aid in elucidating the genesis of mucinous tumors occurring simultaneously in the ovary and appendix, accompanied by psuedomyxoma peritonei. Five such cases were immunostained with anti-CK-7, and all showed a concordant staining pattern of the appendiceal, ovarian, and peritoneal lesions. Two cases showed a negative reaction for CK-7 and thus would appear to represent ovarian and peritoneal metastases from an appendiceal primary tumor. Three cases were CK-7 positive, and the nature of these mucinous lesions remains open to debate; they may either represent independent primary tumors or originate from the appendix. For comparison, five Stage I mucinous borderline tumors of the ovary and their normal appendices were also stained with anti-CK-7. These ovarian tumors were all CK-7 positive, whereas the appendices were negative. It is concluded that CK-7 is capable of distinguishing a group of tumors that can reliably be classified as primary appendiceal neoplasms metastatic to the ovaries and peritoneum.