The case of a patient with an endometrioid carcinoma arising in endometriosis of the rectum is presented. This lesion grossly appeared to be a carcinoma arising from the rectal mucosa until microscopic examination of the specimen was completed. The endometrial implant continued to grow and eventually degenerated into a carcinoma in spite of bilateral salpingoophorectomy. This was believed to be due to maintenance of the patient on diethylstilbestrol. This case is believed to be the ninth reported case of carcinoma arising in an endometrial implant in the colon or rectum.