This paper reviews the consistency of the relation between increasedphysical activity and reduced risk of colon cancer, estimates the potentialprevention benefit from increasing population levels of physical activity,and considers social strategies to increase activity levels. The publishedliterature was reviewed systematically and supplemented by MEDLINE searchesthrough March 1997. Studies that reported a measure of physical activity andoutcomes of colon cancer or colorectal cancer were included. We excluded thefirst report of a study that was expanded subsequently by extended follow-up,and any study that did not report the methods for measurement of physicalactivity. Data were extracted including details on study size, methods ofclassifying physical activity, and outcomes. A consistent inverse relationwas observed such that increased physical activity was associated withreduced risk of colon cancer. About a 50 percent reduction in incidence wasobserved among thos e with the highest level of activity across numerousstudies that used different measures of activity (occupational orleisure-time activity). This association persisted in studies usingmultivariate analyses to control for diet and other known or suspected riskfactors for colon cancer. Risk reduction was attenuated in those studies thatcombined colon and rectal cancer. This review indicates that greaterattention should be placed on social strategies to increase physical activityas a means of preventing colon cancer.