Ovarian epithelial inclusions lined by mucinous epithelium are rare and of uncertain origin. Ovaries containing such inclusions were studied in 42 women. The inclusions were divided into 3 groups: serous epithelial lined with typical ciliated morphology but with distinct basophilic cytoplasmic mucin in some or all of the lining cells, those lined by typical mucinous epithelium, and those lined by a combination of typical mucinous epithelium and serous epithelium. The mean patient age was 61.5 years. Pure mucinous inclusions were found in 27 patients, serous-type inclusions with cytoplasmic mucin in 20, and mixed type in 10. All 3 types of inclusions were found in 1 patient. Two types of inclusions were found in 13. Four patients had associated mucinous neoplasms (1 mucinous cystadenoma, 1 atypical proliferative seromucinous tumor, and 2 seromucinous cystadenomas), and 11 patients (26%) had endometriosis. The fallopian tubes in 4 patients (9.5%) also displayed mucinous metaplasia; this was not significantly different from the 3.1% we found in our previously reported series of unselected tubes from the same population. These findings suggest that mucinous inclusions may arise as a direct metaplastic change in serous-type inclusions. Other possible origins of mucinous inclusions in the ovarian cortex include endometriosis and Brenner (transitional cell) nests. Whether such inclusions can be a source of mucinous ovarian neoplasms as are Brenner tumors and mature cystic teratomas is unknown and may warrant further investigation.