Our model proposes a taxonomy or classification scheme for different measures of health outcome. We divide these outcomes into five levels: biological and physiological factors, symptoms, functioning, general health perceptions, and overall quality of life. In addition to classifying these outcome measures, we propose specific causal relationships between them that link traditional clinical variables to measures of HRQL. As one moves from left to right in the model, one moves outward from the cell to the individual to the interaction of the individual as a member of society. The concepts at each level are increasingly integrated and increasingly difficult to define and measure. AT each level, there are an increasing number of inputs that cannot be controlled by clinicians or the health care system as it is traditionally defined.