Endometriosis is a condition that occurs when endometrial tissue resides outside the uterus. In this case, endometrial tissue attached to and then grew through the wall of the cecum leading to large intestinal bleeding and surgical resection. One of the issues with endometriosis is that it can be mistaken for colonic adenocarcinoma. The histologic features of endometriosis that distinguish it from adenocarcinoma are discussed.