Malignant tumors arising from endometriosis are rare. A frequency of about 1% has been reported with in 80% the ovary, and in 20% extragonadal sites being affected. The most common extragonadal manifestations are the rectosigmoid and the rectovaginal septum. For extragonadal malignant tumors arising from endometriosis, complete resection followed by post-operative radiotherapy, possibly plus adjuvant progestin therapy, is the treatment of choice. Endometriosis-associated ovarian carcinomas are likely to present with lower stage disease and predominantly lower grade tumors. While their treatment follows that of common ovarian cancer, a poorer response to chemotherapy must be considered. As unopposed estrogen replacement therapy has been identified as a risk factor for the development of endometriosis-associated cancer, it is not recommended for hormone replacement therapy in women with a history of endometriosis. Loss of heterozygosity and mutations of the PTEN tumor suppressor gene may be early events of tumorigenesis. Endometriosis and its malignant transformation, perhaps, may serve as a suitable model in this regard. According to recent studies, endometriosis is associated with an increased relative risk of non-Hodgkin lymphoma.