Therapeutic exercise in patients with severe COPD improves useful exercise performance-endurance. This results in an improved quality of life, decreased hospitalization, and decreased costs. Therapeutic exercise is only one modality of the general care of the patient with COPD who often requires the coordinated skills of a multidisciplinary health care team to achieve maximal rehabilitation. These benefits will accrue to most but not all patients receiving exercise therapy. In patient with severe COPD, therapeutic exercise does not improve pulmonary, hemodynamic, or peripheral (muscle) indices. Such patients are rarely able to reach a VO2max or achieve the necessary threshold of exercise level, duration, and frequency required for cardiopulmonary conditioning. Such patients do improve mechanical skills in the trained muscles and benefit psychologically. Supplemental oxygen will improve exercise performance in patients with COPD. This is particularly evident in those who become hypoxemic or who develop overwhelming dyspnea with exercise. Others who benefit when studied cannot be otherwise identified. The respiratory muscles in severe COPD are subject to an increased work of breathing because of increased flow resistance in the airways and a compromise in respiratory muscle contractility owing to the shortened length-tension relationship of the diaphragm because of lung hyperinflation. In addition, they are weak. As in normal persons, the inspiratory muscles are of paramount importance. The respiratory muscles in severe COPD can become fatigued. Hyperpneic eucapnea or inspiratory resistive training can improve respiratory and general exercise endurance. Therapeutic exercise should be prescribed only after a solid general care program is instituted: discontinuation of smoking; moderation in activity; patient and family education; immunization; bronchodilators; early treatment of infections; corticosteroids if efficacious; long-term continuous O2 if indicated; and general rehabilitation. In patients with mild disease and few symptoms, encouragement to participate in general conditioning activities may suffice.(ABSTRACT TRUNCATED AT 400 WORDS)