Epidemiologic data indicate that the relationship between obesity and obstructive sleep apnea (OSA) is largely explained by variation in neck size. Fat deposits in the neck may predispose to upper airway (UA) occlusion during sleep by altering the mechanical properties of the UA, particularly at the level of the velopharynx (VP). To determine whether effective compliance (CEFF) of the VP is related to obesity in the neck, we have performed anthropometrics, polysomnography, and dynamic UA videoendoscopy in 18 patients with OSA and eight nonapneic control subjects. Following topical anesthesia (cocaine 4%) to one nasal passage, subjects were examined with a thin fiberscope (outer diameter [OD] 3.8 mm) while awake, supine, breathing exclusively through the nose, during a maximal vital capacity (VC) maneuver against an unoccluded airway. UA cross-sectional area (CSA), anteroposterior (AP) and lateral (L) diameters were measured from stored images. CEFF was expressed as the ratio of CSAVP in expiration (E) to inspiration (I) during the maximal VC maneuver (expiratory CSAMAX/inspiratory CSAMIN), and airway shape as AP/L diameter. The OSA patients were divided into two groups based on median neck circumference (NC). CEFF of the VP was greater in OSA patients compared with control subjects (CEFF 2.0 +/- 0.8 versus 1.1 +/- 0.5; p < 0.001). CEFF was greater in the large NC group compared with both the small NC group (p < 0.05) and control subjects (p < 0.01), but the difference between the small NC group and control subjects was not significant. CEFF was related to NC (r = 0.63; p = 0.001; n = 26), body mass index (r = 0.54; p < 0.01, n = 26), and apnea-hypopnea index (r = 0.43; p < 0.05; n = 23). The change in CSAVP during the maximal VC maneuver was mainly due to inspiratory narrowing. Obese patients with large necks have a more collapsible velopharynx during wakefulness, which may predispose to upper airway obstruction during sleep.