Colonoscopy is the initial examination for patients with manifest or occult rectal bleeding, inflammatory bowel disease and colorectal polyps. In addition to polypectomy, colonoscopy is useful in decompression of adynamic colonileus and laser palliation of intractable tumours. In adenoma patients controls should be limited to high risk patients, i.e. those with large and multiple tubular adenomas, villous adenomas, multiple hyperplastic polyps (> 30), and first degree relatives of patients with colorectal carcinomas. Control after radical surgical treatment of colorectal cancer is offered during the first two years after the operation and to persons younger than 40 years. The efficacy of control programmes for hereditary nonpolyposis colorectal cancer families have to be evaluated in controlled series. Rectosigmoidoscopies could probably replace some total colonoscopies to examine ulcerative colitis patients for cancer, since cancer usually occurs in the distal colon. Complications are rare after diagnostic and therapeutic colonoscopies, but perforation, bleeding and injury to surrounding organs can be experienced.