There is an important distinction between worth and affordability which remains largely unrecognized. It is widely supposed that if an economic analysis shows that the benefits of a service exceed its costs, failure to fund it is irrational and inefficient. By means of a simplified model of a health care system, it is shown that although an excess of benefits over costs is a necessary precondition for providing a service, it is by no means sufficient. If society is to make the best use of its resources in health care, worthwhile services--those which make a social 'profit'--must be compared with other such services. Since the resources available to any health care system will always be limited, it is likely that not all services whose benefits exceed their costs can be afforded, because the budget has already been committed to those worthwhile services which yield higher benefits per unit cost.