Anatomically, colon and rectum are neighbors. Doctors tend to place colon cancer and cancer of the rectum together, hence the term 'colo-rectal'. Typically, a patient with colorectal cancer complains of a change in bowel habits with or without  passing of blood stained stools.

Cancer of the large bowel (colon and rectum) usually occur after the age of 50, are slightly more frequent in women than in men, and are almost as common as lung cancer in the United States. The high incidence of colorectal cancer in the western world, as contrasted with the low incidence in Japan and rural Africa, suggests that a diet high in refined carbohydrates and beef and low in roughage may be a causative factor.

The risk of large bowel cancer is increased in many colonic diseases. These are chronic ulcerative colitis, diverticulosis,  villous adenomas, and especially  familial polyposis of the colon. People who have inhaled asbestos fibers or who have been irradiated are more likely than others to develop colorectal cancer.

Most lesions of the large bowel are adenocarcinomas; one half arise in the rectum, one fifth in the sigmoid colon, approximately one sixth in the cecum and ascending colon, and the rest in other sites. Rectal tumors may cause pain, bleeding, and a feeling of incomplete evacuation; they may spread slowly through lymphatic channels and veins.

Sometimes the cancerous growths prolapse through the anus. Ring-shaped growths in the sigmoid and descending colon constrict the intestinal lumen, causing partial obstruction and the production of flat or pencil-shaped stools. Cancerous growths in the ascending colon are usually large growths that may be felt by doctor's palm over the abdomen on physical examination. Many patients have severe anemia, nausea, and alternating constipation and diarrhea.

The diagnosis of colorectal cancer is based on digital rectal examination, testing for blood in the stool, proctosigmoidoscopic examination of the sigmoid, and x-ray studies of the gastro-intestinal tract using barium enema. Suspicious polyps may be removed for histologic study, often through a sigmoidoscope or colonoscope or by a laparotomy.

Surgical treatment of colorectal cancer may involve a wide resection of the lesion, the surrounding colon, and the attached tissues, with an end-to-end anastomosis of the remaining intestinal segments whenever possible. Cancer of the lower two thirds of the rectum usually require removal of the entire rectum by abdominoperineal resection and the creation of a permanent colostomy.  Irradiation may be administered preoperatively and postoperatively as palliative therapy for inoperable tumors. Chemotherapy with 5-fluorouracil infused intraluminally in the bowel at surgery and intravenously after surgery may be used as adjunctive treatment.