This syndrome consists essentially of respiratory failure in patients with normal lungs and normal chest walls. In most individuals, there is normal conscious control of ventilation but failure of automatic control, particularly during sleep. Syndromes of alveolar hypoventilation throw light on the complexity of the factors that may underlie the development of respiratory failure in a wide variety of circumstances. Hypoventilation may occur in association with obesity and as a consequence of obstructive sleep apnea.