Although the sleep apnea hypersomnolence syndrome (SAHS) is now a widely recognized disorder, little is known about the timing and pathogenesis of the abnormal airway mechanics that lead to airway occlusion. In order to characterize the changes in airway resistance and to determine if abnormal airway mechanics are confined to inspiration or expiration in SAHS, we separately determined the inspiratory and expiratory resistances on the 2 breaths preceding occlusive apnea in 7 patients. We observed that 6 subjects experienced a significant increase in expiratory resistance, and 5 subjects had a significant rise in inspiratory resistance from the second to the first breath before apnea. For the group, the median expiratory resistance increased from 11.5 to 18.6 cm H2O/L/s (p less than 0.05), and the median inspiratory resistance increased from 10.5 to 17.5 cm H2O/L/s (p less than 0.05) on the 2 breaths before apnea. We conclude that airway abnormalities occur during both the expiratory and the inspiratory phases of ventilation on breath(s) leading to apnea and, therefore, the loss of airway patency is not solely an inspiratory phenomenon. It is also apparent that patients with SAHS thus experience both inspiratory and expiratory resistive loading prior to apnea.