Obesity, particularly severe obesity, affects resting and exercise-related respiratory physiology. Severe obesity classically produces a restrictive ventilatory abnormality characterized by reduced expiratory reserve volume. Obstructive ventilatory abnormality may also be associated with abdominal obesity. Decreased peak work rates are usually seen among obese subjects in a setting of normal or decreased ventilatory reserve and normal cardiovascular response to exercise. Weight loss may reverse many adverse physiologic consequences of severe obesity on the respiratory system.