Physical inactivity has been related to the occurrence of coronary heart disease, hypertension, diabetes mellitus, and osteoporosis. The literature was reviewed to determine what is and what is not known about the efficacy and safety of physical activity in each of these conditions. Although there is a transient increase in the risk of sudden cardiac death during vigorous activity, there is mounting evidence that habitual vigorous activity is associated with an overall reduced risk of coronary heart disease. It is unlikely that this association merely reflects the "selection" that results from sick persons who tend to be less active. Several studies suggest that physical activity may be related to the prevention and control of hypertension, diabetes mellitus, and osteoporosis. However, additional research is needed to make explicit the risks and benefits of physical activity in each of these conditions. Finally, future efforts should determine the type, intensity, frequency, and duration of activity required to maximize the benefits and minimize the hazards of physical activity. The public health and clinical significance of these questions requires that they be examined in the most rigorous manner feasible.