A malignant tumour arising from the inner wall of the large intestine. The third leading cause of cancer in males, fourth in females in the u.s. Risk factors include heredity (family history), colon polyps, and long-standing ulcerative colitis. most colorectal cancers develop from polyps. Colon polyp removal can prevent colorectal cancer. Colon polyps and early cancer can have no symptoms so regular screening is important. Diagnosis of colorectal cancer can be made by barium enema or by colonoscopy with biopsy confirmation of cancer.