Telephone interview data from aggregated state surveys showed that about 21 percent of the U.S. adult population expends greater than or equal to 3 kcal/kg-day in vigorous leisure-time exercise. Three kilocalories per kilogram-day is equivalent to the amount commonly recommended to maximally reduce the incidence of coronary heart disease. Approximately 36 percent of the U.S. population reported no vigorous leisure-time exercise. Men, younger persons, and the more highly educated were most likely to expend greater than or equal to 3 kcal/kg-day, but for no subgroup did the rate exceed 30 percent. People who did not smoke, were not obese, and who did wear seat belts are also more likely to expend energy in vigorous leisure-time exercise. The prevalence of alcohol misuse is similar for all exercise categories. The proportion of people who expend greater than or equal to 3 kcal/kg-day is unrelated to self-reported occupational physical effort. Given the established and presumed benefits of physical activity, a substantial portion of the U.S. population would probably benefit from regular, vigorous, leisure-time exercise.