THE US health care delivery system is changing rapidly, dominated mainly by the shift from fee-for-service to managed care medicine. What are the implications for the practice of medicine as a result of the shift from patient-based to population-based medicine? As resources directed to health care are reduced, how will the trade-off between cost and quality be altered? Will quality even remain on the agenda as health system reform proceeds? Health services research over the past 25 years has produced many findings relevant to these issues. For example, losing or acquiring health insurance affects people's health; but if economic incentives are used to alter the amount of care consumed, then the use of clinically based tools is required to avoid the approximately equal decline in necessary and lessthan-necessary care. To keep quality of care on the agenda, physicians should be provided both information about the use of specific health services