This paper reviews the consistency of the relation between increased physical activity and reduced risk of colon cancer, estimates the potential prevention benefit from increasing population levels of physical activity, and considers social strategies to increase activity levels. The published literature was reviewed systematically and supplemented by MEDLINE searches through March 1997. Studies that reported a measure of physical activity and outcomes of colon cancer or colorectal cancer were included. We excluded the first report of a study that was expanded subsequently by extended follow-up, and any study that did not report the methods for measurement of physical activity. Data were extracted including details on study size, methods of classifying physical activity, and outcomes. A consistent inverse relation was observed such that increased physical activity was associated with reduced risk of colon cancer. About a 50 percent reduction in incidence was observed among those with the highest level of activity across numerous studies that used different measures of activity (occupational or leisure-time activity). This association persisted in studies using multivariate analyses to control for diet and other known or suspected risk factors for colon cancer. Risk reduction was attenuated in those studies that combined colon and rectal cancer. This review indicates that greater attention should be placed on social strategies to increase physical activity as a means of preventing colon cancer.