Ovarian adenocarcinomas of endometrioid type were studied by different histological and histochemical technics to determine their morphologic criteria, the development of these tumors as well as their relationship to different conditions of the endometrium. Mucin secretion was evident as was cellular polymorphism on the ultrastructural level. Surface analysis revealed an endometrium-like pattern with cells showing cilia as well as numerous microvilli, different from that of other ovarian epithelial surface tumors. Ovarian endometrial lesions showed pattern as well as occasional derangement of surface structures in histologically regular epithelium.