An obese patient with a ten year history of respiratory failure presented with insomnia and marked daytime somnolence. Respriatory failure had been attributed to obesity, respiratory centre insensitivity to carbon dioxide, and to diffuse airways obstruction. To investigate the possible role of episodic apnoea with frequent nocturnal arousals, continous recordings were obtained during sleep of arterial oxygen saturation, oesophageal pressure and the motions of the rib-cage and abdomen/diaphragm. Repeated episodes of hypoventilation and profound hypoxaemia were found which were due to intermittent obstruction of the upper airway rather than to cessation of breathing efforts. During the episodes of hypoxaemia, values of arterial O2 tension fell to as low as 24 mmHg. Episodic hypoxaemia was relieved but not abolished, by the use of a collar, designed to hold the mandible forward. Previous reports indicated that recognition of intermittent obstruction of the upper airway during sleep and treatment by a permanent tracheostomy, resulted in a significant long-term imporvement of pulmonary and cardiac function and relief of insomnia and day-time somnolence. When tracheostomy is inadvisable, as in the present patient, it is hoped that similar long-term benefits will result from a supportive collar.