The severity of chronic obstructive pulmonary disease (COPD) and patients' response to therapy are difficult to assess. The traditional measure, spirometry, correlates poorly with important clinical features of the disease, such as survival and quality of life (QOL). Moreover, COPD has recently been recognized as a systemic disease, and its systemic manifestations, such as weight loss and muscle weakness, are only poorly related to lung function. Therefore, although lung function remains an important outcome, other outcomes must be included in any overall assessment of disease severity or response to interventions. Examples include refinements of spirometry, such as measurement of FEV6 and inspiratory capacity; functional outcomes, such as dyspnea indexes and exercise tests; and global-clinical outcomes, such as QOL questionnaires and assessment of frequency and severity of acute exacerbations. For scoring disease severity, making a prognosis, or determining the outcome of novel interventions, composite measures need to be developed that take into account as many aspects of COPD as practicable.