Study Objectives: Current evidence suggests that the pathological mechanisms underlying obstructive sleep apnea (OSA) are altered with age. However, previous studies examining individual physiological traits known to contribute to OSA pathogenesis have been assessed in isolation, primarily in healthy individuals. Design: We assessed the four physiological traits responsible for OSA in a group of young and old patients with OSA. Setting: Sleep research laboratory. Participants: Ten young (20-40 y) and old (60 y and older) patients with OSA matched by body mass index and sex. Measurements and Results: Pharyngeal anatomy/collapsibility, loop gain (LG), upper airway muscle responsiveness/gain (UAG) and the respiratory arousal threshold were determined using multiple 2- to 3-min decreases or drops in continuous positive airway pressure (CPAP). Passive pharyngeal anatomy/collapsibility was quantified as the ventilation at CPAP = 0 cmH(2)O immediately after the CPAP drop. LG was defined as the ratio of the ventilatory overshoot to the preceding reduction in ventilation. UAG was taken as the ratio of the increase in ventilation to the increase in ventilatory drive across the pressure drop. Arousal threshold was estimated as the ventilatory drive that caused arousal. V-eupnea was quantified as the mean ventilation prior to the pressure drop. In comparison with younger patients with OSA, older patients had a more collapsible airway (ventilation at 0 cmH(2)O = 3.4 +/- 0.9 versus 1.5 +/- 0.7 L/min; P = 0.05) but lower V-eupnea (8.2 +/- 0.5 versus 6.1 +/- 0.4 L/min; P < 0.01) and a lower LG (5.0 +/- 0.7 versus 2.9 +/- 0.5; P < 0.05). The remaining traits were similar between groups. Conclusions: Our data suggest that airway anatomy/collapsibility plays a relatively greater pathogenic role in older adults, whereas a sensitive ventilatory control system is a more prominent trait in younger adults with obstructive sleep apnea.