The Medical Outcomes Study was designed to (1) determine whether variations in patient outcomes are explained by differences in system of care, clinician specialty, and clinicians' technical and interpersonal styles and (2) develop more practical tools for the routine monitoring of patient outcomes in medical practice. Outcomes included clinical end points; physical, social, and role functioning in everyday living; patients' perceptions of their general health and well-being; and satisfaction with treatment. Populations of clinicians (n = 523) were randomly sampled from different health care settings in Boston, Mass; Chicago, Ill; and Los Angeles, Calif. In the cross-sectional study, adult patients (n = 22,462) evaluated their health status and treatment. A sample of these patients (n = 2349) with diabetes, hypertension, coronary heart disease, and/or depression were selected for the longitudinal study. Their hospitalizations and other treatments were monitored and they periodically reported outcomes of care. At the beginning and end of the longitudinal study, Medical Outcomes Study staff performed physical examinations and laboratory tests. Results will be reported serially, primarily in The Journal.