Tamoxifen therapy may result in a variety of endometrial proliferative lesions, including adenocarcinoma, and as recently suggested, proliferative changes within endometriosis. This report describes an endometrioid adenocarcinoma arising in ovarian endometriosis in a patient taking tamoxifen. There were also foci of benign and borderline endometrioid adenofibroma in the same ovary and a synchronous endometrioid endometrial adenocarcinoma in the uterus. The spectrum of benign, borderline, and malignant endometrioid neoplasia arising within endometriosis suggests that tamoxifen, as a result of its estrogenic effects, may cause proliferative and, in rare instances, malignant changes in endometriosis.