Patients with obstructive sleep apnea (OSA) and heavy snorers without apnea both show intrathoracic suction pressures during sleep that exceed their static upper airway closing pressures. Complete airway occlusion, however, occurs only in the former patient group. We hypothesized that the kinetic properties of the airflow would be different in these two types of patients because of differences in upper airway morphology. The pharyngeal computed tomography (CT) was used to measure the cross-sectional areas of the upper airways in 15 patients with OSA, 25 nonapneic heavy snorers, and 14 control subjects while they were awake. Nocturnal breathing was monitored with the static charge-sensitive bed (SCSB). The patients with OSA had a narrower airspace at the velopharyngeal (VP) level than the controls (p less than 0.01); the nonapneic snorers did not differ from the other groups. At the tongue base (TB) and the hyoid bone (HB) levels there was no difference between the OSA and the control groups, but the nonapneic snorers had narrower airways at both of these levels compared with control subjects (p less than 0.01) and at the hyoid bone level compared with the OSA group (p less than 0.05). The VP/HB ratio was the parameter that best distinguished the patients with OSA from the nonapneic snorers (lower in the OSA group, p less than 0.001). We suggest that airway collapse during sleep is favored by a narrow velopharynx associated with large hypopharynx. Some heavy snorers may not have an oropharyngeal collapse because the peak inspiratory suction pressure could already be damped down at the level of the relatively narrow hypopharyngeal airways.