Three women had endometriosis that involved the rectosigmoid colon; their clinical presentation suggested primary colonic malignant neoplasm. Intestinal obstruction, weight loss, and, in two patients, rectal bleeding with radiologic evidence of a mass lesion that involved the rectosigmoid were present at initial evaluation. All patients eventually underwent colonic resection as definitive therapy. Endometriosis of the pelvic colon may mimic primary intestinal disease, mistakenly suggesting malignant neoplasm. Such symptoms in a young woman should prompt a search for endometriosis, which is a more likely diagnosis. Adequate therapy frequently requires surgical intervention.