To help define the place of nasal continuous positive airway pressure (n-CPAP) treatment in a general sleep apnea population we studied 12 of 13 consecutively diagnosed patients with obstructive sleep apnea syndrome (OSAS). The immediate effects of n-CPAP were tested during a nighttime sleep study divided into two approximately equal parts (control and n-CPAP). Nasal CPAP of 5-10 cm H20 decreased apnea index (apneas per hour of sleep) (control 35.1, n-CPAP 5.7; p < 0.001) and significantly improved oxyhemoglobin saturation (Sao2). The effect was independent of body weight and the presence of cardiorespiratory complications. However, in three patients with lung disease and markedly elevated Paco2, significant sleep-related hypoxemia persisted at the relatively low pressures required to open the upper airway. Long-term home-based n-CPAP was offered to 11 patients. (One patient was considered unsuitable because of persisting profound sleep-related hypoxemia.) Seven patients consented and were followed for periods ranging from 1 to 18 months. All patients reported dramatic reversal of daytime hypersomnolence; three complained of minor nasal stuffiness but compliance was good and only one stopped using the mask (after 12 months). Apnea index decreased following home use of n-CPAP (before 35.9, after 18.1; p < 0.01) but overall respiratory instability (apnea + hypopnea) and Sao2 were not significantly improved. It is concluded that n-CPAP is a highly effective means of preventing upper airway occlusion in OSAS and, except for some patients with coexisting lung disease, it totally reverses the accompanying gas exchange disturbance. Long-term home-based n-CPAP therapy is acceptable to a majority of patients, is free of serious side effects, and appears to result in a partial reversal of the underlying breathing disorder.