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During sleep, mild reduction in inspiratory airflow is associated with snoring, whereas obstructive hypopneas and apneas are associated with more marked reductions in airflow. We determined whether the degree of inspiratory airflow reduction was associated with differences in the collapsibility of the upper airway during sleep. Upper airway collapsibility was defined by the critical pressure (Pcrit) derived from the relationship between maximal inspiratory airflow and nasal pressure. In 10 asymptomatic snorers, six patients with obstructive hypopneas, and 10 patients with obstructive apneas, during nonrapid eye movement sleep, Pcrit ranged from −6.5 ± 2.7 cm H₂O to −1.6 ± 1.4 and 2.5 ± 1.5 cm H₂O, respectively (mean ± SD, p < 0.001). Moreover, higher levels of Pcrit were associated with lower levels of maximal inspiratory airflow during tidal breathing during sleep (p < 0.005). We conclude that differences in …