Background: Adenosarcoma arises most commonly in the uterus. Rarely, it may arise from extrauterine sites, such as the pelvic peritoneum and rectovaginal septum. Two cases of extrauterine Müllerian adenosarcoma arising from pelvic endometriosis are discussed. Cases and Results: Case 1: A 63-year-old woman presented with vaginal bleeding and a rectovaginal nodule. She underwent an extended hysterectomy with resection of a rectovaginal mass. Histology revealed endometriotic tissue. She returned 21 months later with abdominal distension, and underwent resection of large pelvic abdominal masses. Histology showed an adenosarcoma with sarcomatous overgrowth. Her disease progressed despite chemotherapy, and she died 5 months later. Case 2: A 70-year-old woman with previous breast cancer presented with urinary retention. Computed tomography showed a 13-cm pelvic mass. She underwent total abdominal hysterectomy and salpingo-oophorectomy. Intraoperatively, dense pelvic endometriosis was seen. Histology showed a stage 1C borderline endometrioid adenofibromatous tumor. She also underwent resection of a 10-cm incidentally discovered pelvic mass 18 months later. Histology showed a low-grade adenosarcoma arising from pelvic endometriosis. She was started on letrozole, and remained well postoperatively. Conclusions: Compared to uterine adenosarcoma, an extrauterine location confers a higher mortality and recurrence risk. Higher stromal grade or sarcomatous overgrowth worsen the prognosis, whereas an association with endometriosis is protective. The mainstay of treatment is surgery. Some surgeons recommend fertilitysparing surgery when there is no sarcomatous overgrowth, while others advocate radical surgery with adjuvant chemotherapy or radiotherapy. Further research is needed to define screening, treatment, and follow-up better. Clinicians should consider this rare diagnosis in any woman presenting with a pelvic mass with a history of endometriosis. (J GYNECOL SURG 35:194