Eleven adult men with sleep apnea underwent nocturnal polysomnography on two successive nights. The first study, done without NCPAP, served as the control. The second (treatment) was done with the application of 7.5 to 15 cm H2O nasal continuous positive airway pressure (NCPAP). A subjective sleepiness index (SSI) was noted upon awakening from each night of polygraphic recording. During the control night, the mean frequency of apnea episodes/sleep hr was 35.95 +/- 4.5 SE, and the mean duration was 28.68 +/- 2.7 sec. Mean frequency of disorder of breathing (DOB) episodes/sleep hr was 19.25 +/- 6.2 and mean duration of DOB episodes was 23.1 +/- 2.8 sec. During the treatment night, all obstructive apnea episodes were abolished. During the control night, the mean decrease in arterial oxygen saturation during obstructive apnea episodes was 11.2 +/- 1.9 percent and the mean lowest saturation was 67.6 +/- 4.0 percent. NCPAP eliminated arterial oxygen desaturation. While 44.5 +/- 5.7 percent of total sleep time was spent in either apnea or disordered breathing during the control night, NCPAP decreased this to 0.73 +/- 0.3 percent. In addition to the improvement in respiration during sleep, SSI decreased from a mean of 3.73 +/- 0.49 after the control night to 1.64 +/- 0.24 after treatment, reflecting an improvement in daytime hypersomnolence. We conclude that nasal CPAP is effective in eliminating obstructive apnea episodes, and results in a marked decrease in daytime hypersomnolence after one treatment night.