In order to explore the efficacy of expiratory positive airway pressure (EPAP) in the prevention of sleep-induced respiratory abnormalities, we studied 9 patients with typical presentation of hypersomnia-sleep apnea syndrome 1 wk apart, without and with 10 cm water of EPAP. We found that EPAP significantly reduced apnea index (p less than 0.01), maximal and mean duration of apneas (p less than 0.01) and desaturation index (p less than 0.001), and significantly improved mean and minimal oxygen saturation (p less than 0.01) and awake supine arterial oxygen saturation (p less than 0.01) in all 9 patients. We also found that EPAP significantly decreased the relative time spent in Stages 1 and 2 NREM sleep (p less than 0.01) and increased the relative time spent in Stages 3 and 4 NREM sleep (p less than 0.01). These results indicate that EPAP reduces frequency and duration of apneas and the degree of nocturnal oxygen desaturation and improves sleep quality in patients with hypersomnia-sleep apnea syndrome.