In our health care environment, managed care, capitated payments, and a growing national information infrastructure are becoming major forces that shape decisions about how medical resources are organized and used. Health care decision makers prize medical effectiveness information based on patient care data that are linked with other data, such as costs. Obtaining uniform, accurate patient care data requires determining conditions of access to the data and standards. Under the National Information Infrastructure Initiative, principles have been developed to guide users and providers of individually identifiable data, and U.S. standards developers are at work to improve the definitions and electronic transmission of patient care data. When aggregated, these data can serve many uses, including improving quality of care. Their use for computing clinical performance measures, such as the Health Plan Employer Data Set (HEDIS), and a study that classifies such sets are highlighted.