The effect of posture on upper airway dimensions was assessed for two reasons. First, some patients with untreated sleep apnea/hypopnea syndrome (SAHS) report they sleep better sitting upright. Second, to allow comparison of the differing techniques used to determine the site of maximal airway narrowing in awake patients with SAHS, as some are carried out in the erect and others in the supine posture. Lateral cephalometry was therefore carried out in 33 nonsnoring normal subjects and in 29 patients with obstructive SAHS (mean apneas plus hypopneas, 46 per hour; range, 17 to 103). In both normal subjects and patients, uvular width was increased (p < 0.05) in the supine posture, and this was associated with significant narrowing of the retropalatal airway in the patients with SAHS (erect, 5.0 ± SD 2.6 mm; supine, 3.6 ± 2.8 mm; p < 0.01). In both normal subjects and patients, the retroglossal hypopharynx widened (p < 0.05) in the supine posture (e.g., in patients with SAHS, posterior airway space was: erect, 11.5 ± 4.5 mm; supine, 13.4 ± 4.8 mm; p = 0.003). In the supine posture there was anterior movement of the hyoid and neck flexion in both groups. However, a study of the effect of neck flexion in the erect posture showed that neck flexion produced no changes in airway caliber. Thus, posture is an important determinant of upper airway dimensions.