This randomized controlled trial is designed to determine whether screening for occult blood in the stool will reduce mortality from colorectal cancer. When the study began in 1975, a cohort of 45,000 individuals between 50 and 80 years of age, residing in the state of Minnesota, with no prior history of colorectal cancer was randomized into three groups by age, sex, and geographic region of the state. Test groups submit Hemoccult slides annually or every other year; the control group does not submit slides. Participants whose slides are positive are evaluated at the University of Minnesota Hospitals using the specified diagnostic protocol. All participants are asked to complete a short annual questionnaire concerning their health status. The study is monitored by an advisory group with expertise in statistics, epidemiology and gastroenterology. The study will continue until a significant difference in mortality is observed between screened and control groups or until it is clear that no significant difference will emerge.