The location of upper-airway obstruction during sleep has been predicted by using waking measures that include physical examination and endoscopic Müller's maneuver. However, this prediction remains speculative. To objectively evaluate these clinical measures, 22 patients with severe obstructive sleep apnea had upper-airway studies during nonsedated sleep with solid-state manometry and videoendoscopy. Objective tongue-base obstruction was measured with four methods, each identifying a different aspect of airway collapse during sleep. Manometry identified tongue obstruction during (1) early and (2) late inspiration, and videoendoscopy identified severe collapse without complete obstruction during (3) late inspiration, and (4) expiration. Twenty-eight waking upper-airway characteristics measured at waking clinical and endoscopic examination were then compared between patients with and without tongue-base segment obstruction to identify characteristics that would discriminate tongue-base obstruction. The results demonstrated that tongue-base/hypopharyngeal pathology during wakefulness was present on clinical examination in 19 of 22 (86%) patients. Objectively, during sleep tongue-base obstruction occurred on manometric early inspiration in 5 of 21 (24%) patients, on manometric late inspiration in 11 of 21 (52%), on endoscopic late inspiration in 14 of 19 (74%), and on endoscopic expiration in 8 of 19 (42%). Patients with tongue-base obstruction during sleep were primarily discriminated on waking examination by nonobstructive upper oropharyngeal features (p < 0.10). Near-total collapse of the tongue base on supine endoscopy was the only characteristic associated with tongue-base obstruction during sleep. Müller's maneuver did not discriminate patient groups. Results indicate that the incidence of tongue-base abnormalities measured during sleep varies significantly depending on the measure used. This variability may contribute to variability in surgical success rates. Identification of airway pathology relevant to sleep-related breathing disorders may require new methods of evaluation and a better understanding of upper-airway biomechanics.