This report presents a case of endometrioid adenocarcinoma arising from endometriosis of the mesenterium of the sigmoid colon following total abdominal hysterectomy and bilateral salpingo-oophorectomy for leiomyoma of the uterus and infiltrating pelvic endometriosis, and hormone replacement therapy. A 62-year-old woman presented with an abdominal tumor. Based on the diagnosis of mesocolonic tumor, sigmoidectomy with lymph node resection was performed. The tumor cells were immunopositive for cytokeratin 7, but negative for cytokeratin 20, and the tumor was histologically diagnosed as endometrioid adenocarcinoma of the mesocolon. Hyperestrogenism has been implicated as a risk factor for the development of cancer from endometriosis. The patient had been receiving high-dose unopposed estrogens for 14 years after a total abdominal hysterectomy and bilateral salpingo-oophorectomy. Physicians should recognize that endometriosis-associated neoplasms are able to cause symptoms or signs such as abdominal and/or pelvic pain, pelvic mass, and vaginal bleeding, especially if the patient has been treated with hormone replacement therapy. It is important to recognize the possibility of tumors arising from endometriosis when evaluating intestinal or mesenteric neoplasms in women, even in the patient who has previously undergone total abdominal hysterectomy and bilateral salpingo-oophorectomy, particularly if the patient has a history of endometriosis and has received hormone replacement therapy.