(FORTUNE Magazine) – When I hear that a patient has been diagnosed with colon cancer or, worse yet, that the cancer has spread outside the colon, I feel a twinge of pain. Almost all colon cancers are preventable. End of story.

Trouble is, there's often another chapter. Each year more than 130,000 Americans develop cancer of the colon and rectum, generally referred to as colorectal cancer. More than 50,000 die, making it second only to lung cancer as a cause of cancer deaths.

Chances of getting colorectal cancer increase with age--roughly 90% of those who develop it are over 50--and it affects men and women equally. Tiny growths called polyps form in the colon; it's believed that over time they can grow and change into cancer. If a polyp or colon cancer is detected early enough, it can be removed and lives can be saved. But because colon cancer rarely produces symptoms in its early stages, we recommend regular screenings in order to discover it.

The first step to avoiding colon cancer is preventing the formation of polyps. Diet is a factor, with red meat and animal fat topping the list of offenders. Alcohol compounds the risk, obesity is a contributor, and smoking may be. Exercise has a strong protective effect against colon cancer, as does hormone replacement treatment in women. Contrary to popular belief, fiber doesn't do much good, but people whose diets are rich in vegetables appear to be at lower risk.

A variety of tests have been developed to screen for colon cancer. All are uncomfortable, but generally not painful. One of the most common approaches combines an annual stool test with a procedure called a flexible sigmoidoscopy every three to five years. A "flex-sig" is a slender, flexible fiber-optic scope that enables the doctor to examine the rectum and the last third of the colon. If a polyp is lodged in the upper two-thirds and bleeds, blood should show up in the stool, which the patient collects through a simple test at home. Some lesions don't bleed, however, and blood in the stool doesn't necessarily mean colon cancer.

A more conclusive screen involves a barium enema every five to ten years, in which a chalky liquid is infused into the colon. An X-ray--preferably with real-time fluoroscopy--then enables the doctor to see a video outline of the colon. Barium enemas are often accompanied by a flex-sig or sometimes a rigid proctoscope, which looks at the last foot of colon. The advantage of the barium X-ray is that it examines the entire colon at a reasonable cost and should detect polyps of clinically significant size. If a polyp is found, we'll schedule a colonoscopy to remove it.

Colonoscopy is the gold standard. Through the use of a colonoscope, a four- to six-foot-long flexible tube that is connected to a videocamera and monitor, a doctor can examine the entire colon (intravenous sedation makes the procedure tolerable). In addition to seeing the entire length of the colon, the doctor performing the colonoscopy can remove polyps and biopsy suspicious lesions during the procedure. The main disadvantage is the cost, which usually runs to more than $2,000. There is also a slight, but serious, risk of perforation.

Virtual colonoscopy, a new procedure in which computer software transforms a CT scan into a 3-D view of the inside of the colon and rectum, is being studied. It's somewhat less accurate than a traditional colonoscopy, and most experts feel it's not quite ready for prime time. Interest in genetic testing, up to now reserved for people at high risk, is likely to increase in the future.

When a polyp is removed, we encourage a repeat colonoscopy in three years to check for new growths. An aspirin a day might help prevent polyps and cancer from returning, but the final answer isn't in yet.

Most doctors urge that testing start at age 50, but earlier isn't a bad idea, especially if you have a family history of polyps or colon cancer or if you're having symptoms such as a change in bowel habits, abdominal discomfort, rectal bleeding, or black stools. Here's the key thing: It matters less which screening regimen you choose and more that you just do it.

It bears repeating: Colon cancer is one of the few conditions that is truly preventable. If it's there and you ignore it, you'll get a rude awakening sooner or later. Don't let fear, the inconvenience of the tests, or embarrassment keep you from doing something that could save your life.

FOR MORE INFORMATION ON THIS TOPIC: Go to mayoclinic.com. Mayo Clinic offers Executive Health Programs in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.