An operational definition of ‘health’ is offered, useful to epidemiologists and others concerned with the allocation of resources for health services. A health status index (HSI) is described, based on social-value judgments. The HSI is the basis for an operational definition of prognosis, which, in conjunction with definitions for population and time, makes it possible to give a quantitative definition of the outcome of health services, and then program effectiveness and population health status. Finally, to illustrate the methodology, the application of such a HSI to the venereal diseases control program for the city of New York is shown. The illustration also makes the point that in a real-life situation, the question of the definition of ‘health’ is only one among many considerations. The complexity of the situation requires the services of the epidemiologist and that of other disciplines, all working together in an inter-disciplinary team.