The axioms of utility theory are restated in terms of health outcomes, and some additional assumptions, consistent with the assumptions implicit in health status index models, are adduced to develop a consistent theory of the utility of health states. On the basis of the axioms and specific assumptions, techniques for measuring the health utility functions of individuals are described, and it is shown how these axioms and assumptions may be used to determine the utility to the individual of health programs that will affect him in various ways.