Histologic material from 42 patients with endometrioid carcinomas of the ovary was reviewed. Ovarian endometriosis was present in 11 cases (26%) and 8 of these patients were postmenopausal. The exact site of transition from benign to malignant epithelium was observed in 4 cases. The clinical characteristics of patients with associated endometriosis were not significantly different from those without this finding except that endometriosis was present only in patients with Grade 1 or Grade 2 carcinomas. These data suggest that ovarian endometriosis in the postmenopausal patient has the potential to undergo malignant transformation and, when detected, should be removed surgically.