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 H2O decreased apnea index (apneas per hour of sleep) (control 35.1, n-CPAP 5.7; p < 0.001) and significantly improved oxyhemoglobin saturation [Sa(O2)]. The effect was independent of body weight and the presence of cardiorespiratory complications. However, in three patients with lung disease and markedly elevated Pa(CO2), 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 Sa(O2) 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.