In 1985, we began a 4-year evaluation of the effects of the diagnosis related groups-based prospective payment system on quality of care for hospitalized Medicare patients. This article provides an overview of the study's background, aims, design, and methods. We used a clinically detailed review of the medical record supplemented by data on postdischarge outcomes drawn from the files of the Health Care Financing Administration and fiscal intermediaries to (1) compare outcomes of care after adjustment for sickness at admission, (2) assess the process of in-hospital care and relationships between processes and outcomes, and (3) assess status at discharge for a nationally representative sample of patients hospitalized before and after prospective payment was implemented.