Screening with primary sigmoidoscopy is encouraged in Scandinavian countries and in the UK. In the USA, annual FOBT or sigmoidoscopy every 5 yars or colonoscopy every 10 years is recommended. New guidelines in the USA also include the use of virtual colonoscopy every 5 years.

Two case-control studies suggest that regular screening of those >50 years with sigmoidoscopy decreases mortality. This type of study is prone to selection biases. A quarter to a third of polyps can be discovered with the rigid sigmoidoscope; half are found with a 35-cm flexible scope and two-thirds to three-quarters are found with a 60-cm scope. Diagnosis of adenomatous polyps by sigmoidoscopy should lead to evaluation of the entire colon with colonoscopy and/or barium enema. The most efficient interval for screening sigmoidoscopy is unknown, but 5 years is often recommended. Case-control studies suggest that intervals of up to 15 years may confer benefit.