A colonoscopy is a procedure to look at the inside of the colon. The colon is the large intestine and the last part of the digestive system. The colon dries, processes, and eliminates the waste left after the small intestine has absorbed the nutrients in food. The colon is about 3 to 5 feet long. It travels from the lower right corner of the abdomen (where the small intestine ends) up to the liver, across the body to the spleen in the upper left corner and then down to form the rectum and anus.

The doctor will use an instrument called the colonoscope to perform a colonoscopy. It is a long (about 5 feet), thin (about 1 inch), flexible fiberoptic camera that allows the doctor to visualize the entire colon. A doctor may order a colonoscopy to investigate many different diseases of the colon.

Colonoscopy is best known for its use as a screening tool for the early detection of colorectal cancer. The doctor may recommend this test if other screening tests such as a manual rectal examination, a fecal occult blood test (a test that detects blood in the feces), or a barium enema (a test in which barium is used to make the colon visible on an X-ray) suggest that further information is needed to make a diagnosis.