The quality of health care is measured today using performance measures that calculate the percentage of people whose health conditions are managed according to specified processes or who meet specified treatment goals. This approach has several limitations. For instance, each measure looks at a particular process, risk factor, or biomarker one by one, and each uses sharp thresholds for defining "success" versus "failure." We describe a new measure of quality called the Global Outcomes Score (GO Score), which represents the proportion of adverse outcomes expected to be prevented in a population under current levels of care compared to a target level of care, such as 100 percent performance on certain clinical guidelines. We illustrate the use of the GO Score to measure blood pressure and cholesterol care in a longitudinal study of people at risk of atherosclerotic diseases, or hardening of the arteries. In that population the baseline GO Score was 40 percent, which indicates that the care being delivered was 40 percent as effective in preventing myocardial infarctions and strokes as our target level of care. The GO Score can be used to assess the potential effectiveness of different interventions such as prevention activities, tests, and treatments.