Primary ovarian mucinous carcinomas are uncommon and usually present as unilateral stage 1 neoplasms. The vast majority are of the so-called intestinal or enteric type and arise from a preexisting intestinal-type mucinous borderline neoplasm. The overall prognosis is good. However, a minor proportion recurs or metastasizes, and this is associated with a poor prognosis. The vast majority of primary ovarian intestinal-type mucinous carcinomas and borderline tumors exhibit a variable degree of positivity with enteric markers and are CA125 negative. The primary purpose of this study was to describe the unusual phenomenon of CA125 immunoreactivity in 8 of 10 metastatic mucinous carcinomas arising after a diagnosis of primary ovarian mucinous carcinoma (n=3) or mucinous borderline tumor of the intestinal type (n=7) in which the primary neoplasms were mostly negative. The reasons underlying this emergent CA125 positivity are not clear, but we speculate it may be because while intestinal type mucinous borderline neoplasms and mucinous carcinomas exhibiting so-called expansile invasion are usually CA125 negative, focal positivity may be seen in areas of infiltrative stromal invasion, which may preferentially metastasize. CA125 positivity in the metastatic neoplasm may result in the pathologist considering an alternative primary site; however, this should not be the case. In our study, we found a 4.2% risk of malignant progression after a diagnosis of primary ovarian mucinous borderline tumor of the intestinal type. In light of this, we favor retaining the term "mucinous borderline tumor," because of this small, but not insignificant, risk of malignant transformation, which in most cases is likely secondary to a focus of invasion being unsampled at the time of reporting the primary neoplasms.