The distinction of metastatic mucinous carcinomas in the ovary from primary ovarian mucinous tumors (atypical proliferative/borderline and carcinoma) can be difficult because of similarities in morphology. We evaluated the immunohistochemical expression of cytokeratins 7 and 20 (CK 7, CK 20), Dpc4 (nuclear transcription factor inactivated in 55% of pancreatic carcinomas), and MUC5AC (a gastric mucin gene) in 57 primary ovarian mucinous tumors (41 atypical proliferative tumors and 16 carcinomas) and 46 metastatic mucinous carcinomas in the ovary. Primary ovarian mucinous tumors were virtually always diffusely positive for CK 7 (98%), Dpc4 (100%), and MUC5AC (98%) and often focally to diffusely positive for CK 20 (68%). Colorectal mucinous carcinomas were diffusely positive for CK 20 (100%) and Dpc4 (89%) and were distinguished from primary ovarian mucinous tumors by their frequent lack of expression of CK 7 and MUC5AC (67% were negative for each marker). Appendiceal carcinomas were diffusely positive for CK 20 (100%) and often negative for CK 7 (71%) but were often positive for MUC5AC (86%) and Dpc4 (100%). When primary ovarian and metastatic colorectal or appendiceal carcinomas shared expression of both CK 7 and CK 20, they could usually be distinguished by the pattern of positivity (diffuse CK 7 and patchy CK 20 in ovarian tumors and patchy CK 7 and diffuse CK 20 in colorectal and appendiceal tumors). Pancreatic carcinomas shared the same pattern of diffuse positivity for CK 7 (100%) and MUC5AC (92%) and focal to diffuse positivity for CK 20 (71%) as primary ovarian mucinous tumors but were negative for Dpc4 in 46%. Loss of Dpc4 expression is useful for distinguishing metastatic pancreatic carcinomas in the ovary from both primary ovarian mucinous tumors and metastatic mucinous carcinomas derived from other sites.