Quantitative estimates indicate that sedentary living is responsible for about one-third of deaths due to coronary heart disease, colon cancer, and diabetes--three diseases for which physical inactivity is an established causal factor. Presumably, if everyone were highly active the death rate from these three disease would be only two-thirds of the current rate. Not everyone will become highly active, however. Assuming smaller increases in physical activity practices, mortality from these three conditions combined could be reduced by as much as 5-6%, or 30,000-35,000 deaths per year. Overall mortality in the United States might be reduced about 1-1.5%. The greatest gains would accrue from strategies that encourage those who report no leisure-time physical activity to do some and that encourage those who are irregularly active to participate in 30 or more minutes of light to moderate activity for 5 or more d.wk-1. Mortality is only one aspect of public health burdens that would be reduced by greater participation in regular physical activity. Quality of life, which we have not attempted to quantify, would also improve.