The effect of obesity as a factor limiting exercise tolerance is described and can be attributed to three mechanisms: (a) the increased metabolic and therefore ventilatory requirement to perform a given work task; (b) the increased metabolic cost of breathing because of interfering chest wall and abdominal obesity and high breathing frequency; and (c) pulmonary insufficiency in response to the high breathing work and lung atelectasis. The role of support and posture minimizing the effect of obesity should be considered when attempting to improve the work tolerance of obese patients. One cannot predict accurately the O2 cost of exercise in the obese patient from the ergometer load because of uncertainties of distribution of the adipose tissue, the uncertain effects on breathing work, and often reduced motor efficiency or skill. The metabolic cost of exercise in the obese patient, therefore, needs to be directly measured; only then can one establish the degree of normality of the physiologic responses to exercise, e.g., heart rate, blood pressure, cardiac output, and minute ventilation.