We present a case of endometrioid adenocarcinoma arising from extragonadal endometriosis 18 years after total abdominal hysterectomy with bilateral salpingo-oophorectomy. After the primary surgery the patient received 11 years of unopposed oestrogen hormone replacement therapy. She presented with symptoms of urinary retention and pelvic mass. Following resection, histopathology identified the mass as an endometrioid adenocarcinoma. The association between persistent endometriosis and the development of endometrial cancer are discussed here together with the risks of unopposed oestrogen in the development of such lesions.