People with inflammatory bowel diseases are at increased risk of colon cancer. These include ulcerative colitis and to a lesser extent Crohn's disease.

Ulcerative colitis is a chronic disease of the large bowel that has episodic behavior and an unpredictable outcome. It is characterized by recurrent inflammation and ulceration of the colon and rectum and primarily involves the mucosa (see Chapter Two). The etiology, or cause, of this disease is unknown. Clinically, the prominent symptoms of ulcerative colitis are diarrhea and rectal bleeding. Other symptoms of attacks are abdominal pain, fever and weight loss. The disease may be treated with medications to control attacks, but ultimately may require surgery to prevent or treat complications and to prevent cancer. The risk of colon cancer from ulcerative colitis is increased when the entire large bowel is involved and increases with the length of time which a person has been known to have had the disease. Since ulcerative colitis is a lifetime disease, the risk of colorectal cancer therefore is higher in people who are first diagnosed with it at a younger age (especially less than twenty-five years). The average age for diagnosis of colorectal cancer in patients with ulcerative colitis is 49 years, compared to 69 years in the general population. Additionally, cancers tend to be multifocal (that is, multiple cancers at different sites in the large bowel) about forty percent of the time. Cancer usually occurs in parts of the bowel that are inflamed by the disease process itself, but can also occur in areas of the bowel not involved by ulceration. Prevention of colon cancer usually involves resection (removal) of the entire large bowel.

Crohn's disease is likewise a chronic and episodic disorder of the bowel, but can affect the entire gastrointestinal tract from mouth to anus. It is distinguished from ulcerative colitis by the fact that it involves the entire wall of the bowel, not just the mucosa. It is usually most prominent in the small bowel, colon and anus. Like ulcerative colitis, the cause is unknown. In some quarters, it is held that Crohn's disease and ulcerative colitis may actually together form part of a larger category of disease. The symptoms of Crohn's disease may mimic those of ulcerative colitis, and include diarrhea, abdominal pain, rectal bleeding, fever, weight loss and in children, growth pattern disturbances. The risk of colon cancer from Crohn's disease is less than that from ulcerative colitis, but may be as high as twenty times the expected incidence in the general population. The average age of cancer occurrence is forty-eight years.