The medical care system is being restructured with the goal of containing rising health care costs, but little attention has been given to what makes a difference to the patient. The National Study of Medical Care Outcomes, known for brevity as the Medical Outcomes Study (MOS), has been designed to expand the debate regarding alternatives for the delivery and financing of health care services. A major feature of the MOS is its emphasis on the patient's perspective. This report focuses on depression, one of four tracer conditions selected for inclusion in the MOS. It deals with 11 features of depression: (1) a definition of the condition; (2) its prevalence in the general population and in the practices of physicians with different training; (3) identification of patients to be enrolled; (4) assessing the severity of the condition; (5) the functional impact of the condition; (6) typical treatments, and their outcomes considered in terms of disease severity and functional outcomes; (7) variation in treatments or outcomes by physician specialty or organization of practice; (8) outcomes relevant for evaluating the treatment of this condition; (9) components of case-mix control; (10) the clinical course of the condition; and (11) policy issues related to the condition and its treatment.