Ovarian epithelial tumors are thought to derive from the surface epithelium of the ovary, which differentiates after invagination of the coelomic mesothelium over the gonadal ridges during early embryonic development. On the putative mullerian potential of this epithelium, endometriosis, including ovarian endometriosis, can be explained by a metaplastic process from the peritoneum. In the present study, we assessed specific changes in the architecture and in the cytological characteristics of human ovarian epithelial cells on the surface and within the inclusion cyst. Grossly normal ovaries were retrieved from 215 patients undergoing incidental oophorectomy and 33 patients with contralateral ovarian tumor. In addition, 75 patients diagnosed with epithelial ovarian cancer and 26 ovarian endometriosis patients undergoing oophorectomy were selected. The incidence of cortical invagination, epithelial papillomatosis and pseudostratification was not different in normal ovaries between pre- and post-menopausal women, whereas inclusion cysts were more frequently observed in post-menopausal women (p<0.05). The occurrence rates of invagination and inclusion cysts were significantly higher in normal ovaries examined after incidental oophorectomy (p<0.05) and in those with contralateral ovarian tumor (p<0.05), respectively. Although serous metaplastic change with cilia was most common and it did not differ before and after menopause, the frequency of mucinous, endometrioid and transitional cell changes was low in the two groups of pre- and post-menopausal women. Mucinous metaplasia was more frequently observed in patients with contralateral ovarian tumor than those without ovarian tumor (12.1% vs. 1.9%; p<0.05). Four (5.3%) of 75 ovarian cancer patients revealed malignant transformed cells from a single layer of normal epithelium covering the ovarian surface or inclusion cyst. Ovarian or extraovarian endometriosis was identified in 16 (21.3%) of 75 epithelial ovarian cancer patients analyzed. An in situ carcinogenesis in the endometriotic cyst was seen in three epithelial ovarian cancers (4.0%). In seven (26.9%) of 26 ovarian endometriosis cases, epithelial cells on the ovarian surface or within the inclusion cyst were histologically changed to endometriotic gland cells. Careful and extensive observations of surgical specimens with ovarian carcinoma and ovarian endometriosis showed that they originated from the ovarian epithelium lining the surface and cortical small cysts.