Efficiency has become of central importance in health care and is seen as wholly laudable. It appears to offer a precise and objective means of evaluating and comparing institutions, practices and individuals, and is a principle that underlies techniques of cost-benefit analysis and other methods of option appraisal. However, there is a need to examine the concept of efficiency and explore the problems of its application within health care. Efficiency is a value laden notion and it cannot be used as a means of making value judgements from purely factual premises. We have to choose the inputs and outputs in our calculations, and what limits to set on their scope. Efficiency calculations may require us to measure what cannot be quantified, and to treat as commensurable what is incommensurable. There are circumstances in which considerations of efficiency are inappropriate or even immoral.