Histologic differentiation of primary ovarian carcinoma from colonic carcinoma metastatic to the ovary may be difficult. Cytokeratin 7 (CK7) and cytokeratin 20 (CK20) immunostaining is usually used, but these markers lack specificity for ovarian and intestinal epithelium, and overlapping results have been reported. Cdx2 is a transcription factor whose expression in normal tissues is limited to the intestinal epithelium. It is also expressed in the vast majority of colonic carcinomas and in a sizeable proportion of cases of gastric, pancreatobiliary, and ovarian mucinous carcinomas. We evaluated Cdx2, CK7, and CK20 expression in 50 ovarian carcinomas (15 serous, 20 mucinous, and 15 endometrioid), 15 colonic carcinomas metastatic to the ovaries, and 20 primary colonic carcinomas. The extent (1-25%/1+, 26-75%/2+, >75%/3+) and intensity (weak/1+, strong/2+) of staining were recorded semiquantitatively. All primary and metastatic colonic carcinomas had diffuse (3+) strong Cdx2 reactivity. All serous and endometrioid tumors were Cdx2 negative, whereas mucinous carcinomas had 1+ or 2+ immunoreactivity. All ovarian carcinomas had strong diffuse CK7 staining, whereas all colonic carcinomas were negative for CK7. CK20 stained diffusely and strongly all primary and metastatic colonic carcinomas and was 1+ or 2+ in all mucinous carcinomas, in 67% of serous carcinomas, and in 33% of endometrioid carcinomas. In conclusion, 1) Cdx2 is a highly sensitive (100%) marker for colonic carcinoma metastatic to the ovary; 2) Cdx2 is more specific than CK20 as it is not expressed by serous and endometrioid carcinomas; and 3) a limited panel of Cdx2 and CK7 helps in distinguishing colonic carcinomas metastatic to the ovaries (Cdx2+/CK7-) from primary ovarian serous (Cdx2-/CK7+), endometrioid (Cdx2-/CK7+), and mucinous (Cdx2+/CK7+) carcinomas.