- Primary extragenital malignant mixed Müllerian tumors (MMMTs) are very rare neoplasms, with only 28 documented cases in the literature. Two cases coexisted with a colonic adenocarcinoma. We report on a primary peritoneal MMMT diagnosed shortly after resection of a colonic adenocarcinoma in an 85-year-old woman who presented with a large omental mass. Microscopic examination revealed a biphasic tumor with malignant carcinomatous and sarcomatous components, confirmed immunohistochemically, consistent with MMMT. Despite optimal debulking and uneventful postoperative recovery, the patient died of her disease shortly after surgery due to recurrent disseminated disease. This is the third case in the literature of primary extragenital MMMT occurring in association with colonic adenocarcinoma. This coexistence may be incidental, but it may also imply a possible linkage between these two tumors.