The effect of weight loss following dietary restriction on disordered breathing on the pharyngeal airway is controversial in patients with obstructive sleep apnea (OSA). We therefore prospectively studied eight patients before and after dietary-induced weight loss. Mean weight loss was 20.6 kg +/- 12.8 SD. After weight loss there were significant improvements in PO2 and PCO2 measured during wakefulness, and in the number of desaturation episodes per hour of sleep, average desaturation per episode, and number of movement arousals. The number of apneas and hypopneas significantly decreased in six of eight patients. There was a significant correlation between body mass index and number of disordered breathing events. Nasopharyngeal collapsibility and pulse flow resistance decreased in awake patients after weight loss. We conclude that moderate weight loss in obese patients with OSA improves oxygenation during both sleep and wakefulness, decreases the number of disordered breathing events in many patients, decreases the collapsibility of the nasopharyngeal airway.