The role of low levels of activity at work and during leisure in the development of cardiovascular morbidity and mortality over the short and long term was studied, based on 24 years of follow up of men in the Framingham Study. Cardiovascular events were examined in 1166 men classified by physical demands of their work and by a 24 hour index of physical activity. In the men, aged 45 to 54 at time of physical activity assessment, there were 303 noncardiovascular, 220 coronary, and 325 cardiovascular deaths during the following 24 years. Overall, cardiovascular and coronary heart disease mortality improved with increasing level of physical activity at all ages including the elderly. Cardiovascular and coronary heart disease mortality were increased in those least active. In morbidity studies, cardiovascular events in general and coronary heart disease in particular was significantly inversely related to physical activity index reflecting 24 hour caloric expenditure. A moderate inverse relation of cardiovascular mortality to the physical demands of the workplace was noted, but was not statistically significant. The benefit, if any, was more apparent in the elderly. No significant relationship between morbidity and physical demand at work was noted. The authors conclude that the unexpected relationship of noncardiovascular mortality rates to physical activity merits further study of possible noxious exposures in work situations where physical exertion is common. For both cardiovascular mortality in general and coronary mortality in particular, physical activity in men remains protective, taking other risk factors into account in a multiple logistic regression.