 The purpose of this study is to see if screening with flexible sigmoidoscopy a flexible viewing tube may reduce large bowel cancer and cancer deaths. The researchers also want to see if the addition of screening for occult blood in stools may contribute further to this aim. Additionally, the researchers also want to see to which extent and in which direction the study may influence overall endoscopic activity in the general population in the screening area and in areas where controlled screening is not established. 