PURPOSE:Endometriosis of the rectovaginal septum can harbor different types of secondary tumors that may involve the rectal wall and protrude into its lumen, thus making diagnosis difficult. Extrauterine low‐grade endometrial stromal sarcoma may rarely arise in endometriosis. The purpose of this article was to present the third case of this association.METHOD:This was a clinicopathologic study.RESULTS:A 42‐year‐old female presented with abdominal pain and fever. Laparotomy revealed a large pelvic mass involving the rectovaginal septum and the colonic wall and which protruded into the lumen forming endoluminal polypoid masses. Concomitant peritoneal nodules and a metastatic paracolic lymph node were also found. Histopathologically, primary endometriotic foci were found in close relationship with an endometrial stromal sarcoma which invaded the rectal wall. The female genital tract had no endometriotic lesions. The patient was treated by surgery and subsequent chemotherapy and was alive and well 20 months later.CONCLUSIONS:Endometriosis and its possible malignant changes should be taken into account in the differential endoscopic diagnosis of rectal masses in females.