Women with pseudomyxoma peritonei (PMP), characterized by multifocal mucinous implants (disseminated peritoneal adenomucinosis), often have synchronous appendiceal and ovarian mucinous tumors. There has been considerable debate as to whether the ovarian tumors are secondary to the appendiceal tumor or are independent primary ovarian tumors; the latter are usually classified as mucinous tumors of low malignant potential (MLMP). It has been reported that cytokeratins (CK) 7, 18, and 20, carcinoembryonic antigen (CEA), and human alveolar macrophage 56 (HAM-56) are useful markers for distinguishing primary ovarian neoplasms from metastases of intestinal origin. Nearly all primary ovarian MLMP tumors and mucinous carcinomas are positive for CK 7, 18, and 20, CEA, and HAM-56, whereas most colorectal adenocarcinomas are negative for both CK 7 and HAM-56 and positive for CK 20 and CEA. Thirteen appendiceal and 14 ovarian mucinous tumors from 14 cases of PMP and 11 primary ovarian MLMP tumors were studied immunohistochemically for expression of CK 7, 18, and 20, monoclonal and polyclonal CEA (mCEA and pCEA), and HAM-56. Of 14 cases of PMP, 10 (71.4%) had identical staining patterns for all antibodies in both the appendiceal and ovarian tumors. For eight of these, the pattern of immunoreactivity was characterized by negative reactions for CK 7 and HAM-56 and positive reactions for CK 18 and 20, mCEA, and pCEA. One additional case for which only the ovarian tumor could be stained had the same pattern. The remaining two cases were also positive for CK 18 and 20 and CEA, but in addition were positive for CK 7. Two cases were discordant only for CK 7 and one case was discordant for both CK 7 and HAM-56. All 11 MLMP tumors were positive for CK 7 and 18, and pCEA. Eight (72.7%) of 11 were positive for HAM-56, mCEA, and CK 20. There was a statistically significant difference in the frequency of immunoreactivity for CK 7 (p = 0.0005) and HAM-56 (p = 0.0002) between the ovarian mucinous tumors in PMP and the MLMP tumors, with the ovarian tumors in PMP tending to be negative for CK 7 and HAM-56, similar to the appendiceal adenomas. Most ovarian mucinous tumors in PMP demonstrate a pattern of immunoreactivity with CK 7, 18, and 20, CEA, and HAM-56 that is identical to the associated appendiceal adenoma and distinct from primary ovarian MLMP tumors, consistent with the interpretation that these ovarian tumors are secondary to the appendiceal tumor.