We hypothesized that subatmospheric intraluminal pressure is not required for pharyngeal occlusion during sleep. Six normal subjects and six subjects with sleep apnea or hypopnea (SAH) were studied during non-rapid-eye-movement sleep. Pharyngeal patency was determined by using fiber-optic nasopharyngoscopy during spontaneous central sleep apnea (n = 4) and induced hypocapnic central apnea via nasal mechanical ventilation (n = 10). Complete pharyngeal occlusion occurred in 146 of 160 spontaneously occurring central apneas in patients with central sleep apnea syndrome. During induced hypocapnic central apnea, gradual progressive pharyngeal narrowing occurred. More pronounced narrowing was noted at the velopharynx relative to the oropharynx and in subjects with SAH relative to normals. Complete pharyngeal occlusion frequently occurred in subjects with SAH (31 of 44 apneas) but rarely occurred in normals (3 of 25 apneas). Resumption of inspiratory effort was associated with persistent narrowing or complete occlusion unless electroencephalogram signs of arousal were noted. Thus pharyngeal cross-sectional area is reduced during central apnea in the absence of inspiratory effort. Velopharyngeal narrowing consistently occurs during induced hypocapnic central apnea even in normal subjects. Complete pharyngeal occlusion occurs during spontaneous or induced central apnea in patients with SAH. We conclude that subatmospheric intraluminal pressure is not required for pharyngeal occlusion to occur. Pharyngeal narrowing or occlusion during central apnea may be due to passive collapse or active constriction.