Severe nocturnal hypoxemia may occur in patients with respiratory muscle weakness caused by neuromuscular disorders. Negative pressure ventilators may be partially effective in these patients but can cause upper airway obstructive apneas. We examined the effectiveness of positive pressure ventilation through a nose mask in preventing nocturnal hypoxemia and compared it with negative pressure systems. We reasoned that nasal positive pressure would provide stability for the upper airway. Five patients with neuromuscular disorders underwent a series of all-night sleep studies under control conditions, negative pressure ventilation, and positive pressure ventilation through a comfortable nose mask. Sleep staging and respiratory variables were monitored during all studies. Daytime awake lung function, respiratory muscle strength, and arterial blood gases were also measured. The severe hypoxemia and hypercapnia that occurred under control conditions were prevented by positive pressure ventilation through a nose mask. Negative pressure ventilation improved NREM ventilation in all patients, but did not prevent severe oxyhemoglobin desaturation, which occurred during REM sleep. Negative pressure ventilation appears to contribute to upper airways obstruction during REM sleep as evidenced by cessation of air flow, reduced chest wall movements, falls in arterial oxyhemoglobin saturation, and hypercapnia. With treatment, daytime PaO2 improved from a mean of 70 to 83 mm Hg, and PaCO2 decreased from a mean of 61 to 46 mm Hg. We conclude that nasally applied positive pressure ventilation is a highly effective method of providing nocturnal assisted ventilation because it stabilizes the oropharyngeal airway.