OBJECTIVE The aim of this paper is to examine whether physical activity plays any role in the prevention of cancer. METHODS To accomplish this, data from published epidemiologic studies on the relation between physical activity and the risk of developing cancer were reviewed. RESULTS The data are clear in showing that physically active men and women have about a 30-40% reduction in the risk of developing colon cancer, compared with inactive persons. Although the data are sparse, it appears that 30-60 min.d(-1) of moderate- to vigorous-intensity physical activity is needed to decrease risk. There is a dose-response relation, with risk declining further at higher levels of physical activity. It is also clear that physical activity is not associated with the risk of developing rectal cancer. With regard to breast cancer, there is reasonably clear evidence that physically active women have about a 20-30% reduction in risk, compared with inactive women. It also appears that 30-60 min.d(-1) of moderate- to vigorous-intensity physical activity is needed to decrease the risk of breast cancer, and that there is likely a dose-response relation. For prostate cancer, the data are inconsistent regarding whether physical activity plays any role in the prevention of this cancer. There are relatively few studies on physical activity and lung cancer prevention. The available data suggest that physically active individuals have a lower risk of lung cancer; however, it is difficult to completely account for cigarette smoking. There is little information on the role of physical activity in preventing other cancers. CONCLUSIONS Physical activity is associated with lower risk of developing certain site-specific cancers, in particular colon and breast cancers.