Using data from the National Health Interview Survey and the RAND Health Insurance Experiment, we estimated the external costs (costs borne by others) of a sedentary life-style. External costs stem from additional payments received by sedentary individuals from collectively financed programs such as health insurance, sick-leave coverage, disability insurance, and group life insurance. Those with sedentary life-styles incur higher medical costs, but their life expectancy at age 20 is 10 months less so they collect less public and private pensions. The pension costs come late in life, as do some of the medical costs, and so the estimate of the external cost is sensitive to the discount rate used. At a 5 percent rate of discount, the lifetime subsidy from others to those with a sedentary life style is $1,900. Our estimate of the subsidy is also sensitive to the assumed effect of exercise on mortality. The subsidy is a rationale for public support of recreational facilities such as parks and swimming pools and employer support of programs to increase exercise.