Colorectal adenocarcinoma is the most common malignancy of the GI tract and the second most common malignant tumor in the United States. Approximately 50% arise in the rectum and rectosigmoid area. Another 25% occur in the sigmoid colon, and the remaining 25% are evenly distributed throughout the remainder of the colon. Nearly all cancers of the colon are adenocarcinomas arising from preexisting adenomas. Most tumors are annular constricting lesions, 2 to 6 cm in diameter, with raised everted edges and ulcerated mucosa. Polypoid tumors are less common, with some having the frondlike appearance of villous carcinoma. Infiltrating scirrhous tumors”common in gastric carcinoma”are rare in the large intestine, unless the patient has ulcerative colitis. The tumor spreads by direct invasion through the bowel wall into pericolonic fat and adjacent organs, through lymphatic channels to regional nodes, and hematogenously through the portal veins to the liver and systemic circulation. Intraperitoneal seeding from a tumor that penetrates the colon wall may also occur. Obstruction is the most frequent complication. Other complications are uncommon but include perforation, intussusception, abscess, and fistula formation. Up to 20% of patients have a second tumor of the large bowel at diagnosis, usually an adenoma or another carcinoma. Approximately 5% of patients will have a second colorectal carcinoma, either simultaneously or diagnosed subsequently. Patients with ulcerative colitis, Crohn disease, familial adenomatous polyposis syndrome, and Peutz-Jeghers syndrome are at increased risk of colon carcinoma.