To further delineate specific staining patterns and refine the differential usefulness of cytokeratin (CK) 7/20 staining, we studied multiple ovarian tumors and primary nongynecologic neoplasms likely to metastasize to the ovary. Immunohistochemical analysis with semiquantitative grading to give quartile scores (0-4) was performed on 127 cases. Subsequent analysis indicated that a more informative diagnostic segregation could be achieved with a biphasic grading system (>50% staining, positive; 50% or less, negative). Lower intestinal tumors were CK7- and usually CK20+, while upper gastrointestinal tumors, including those of pancreatobiliary origin, were mostly CK7+ and CK20-. Serous papillary ovarian tumors were all CK7+ and CK20-. Mucinous ovarian carcinomas were all CK7+ and slightly more often CK20-, whereas the small number of ovarian borderline mucinous tumors studied were the most problematic, with no clear pattern. Multiple different tumor types from all nonovarian gynecologic sites were fairly consistently CK7+ and almost always CK20-. Differential CK staining of mucinous tumors of the female genital tract using CK7 and CK20 is useful for predicting the site of origin, provided samples are adequate in size. The most specific usefulness is the identification of lower gastrointestinal vs "other" neoplasms.