We outline the implementation of a new method of measuring the quality of medical care that counts cases of unnecessary disease and disability and unnecessary untimely deaths. First of all, conditions are listed in which the occurrence of a single case of disease or disability or a single untimely death would justify asking, "Why did it happen?" Secondly, we have selected conditions in which critical increases in rates of disease, disability, or untimely death could serve as indexes of the quality of care. Finally, broad categories of illness are noted in which redefinition and intensive study might reveal characteristics that could serve as indexes of health. We describe how these inth of the general population and the effects of economic, political, and other environmental factors upon it, and to evaluate the quality of medical care provided both within and without the hospital to maintain health and to prevent and treat disease.