Changes in sleep posture have been shown to improve obstructive sleep apnea (OSA). To investigate the mechanisms by which this occurs we assessed upper airway stability in eight patients with severe OSA in three postures (supine, elevated to 30 degrees, and lateral). We used a specially adapted nasal continuous positive airway pressure (nCPAP) mask to measure upper airway closing pressure (UACP) and upper airway opening pressure (UAOP) during non-REM sleep. Statistical comparisons were made between postures using ANOVA for repeated measures. Elevation resulted in a less collapsible airway compared with both the supine and lateral positions (mean UACP: 30 degrees elevation -4.0 +/- 3.2 compared with supine 0.3 +/- 2.4 cm H2O, p < 0.05 and; lateral -1.1 +/- 2.2 cm H2O, p < 0.05). Supine UACP and lateral UACP were not significantly different. Elevation or lateral positioning produced a 50% reduction in mean UAOP (supine 10.4 +/- 3.5 cm H2O compared with 30 degrees elevation 5.3 +/- 2.1, p < 0.05; and lateral 5.5 +/- 2.1 cm H2O, p < 0.05). We conclude that in severely affected OSA patients upper body elevation, and to a lesser extent lateral positioning, significantly improve upper airway stability during sleep, and may allow therapeutic levels of nCPAP to be substantially reduced.