To determine whether the adoption of a more upright sleep posture would improve breathing and gas exchange in patients with obstructive sleep apnea syndrome (OSAS), 13 male patients with OSAS were studied during an all-night polysomnographic study while lying supine or sitting at a 60-degree angle. In the upright posture, the frequency of obstructive apnea was decreased (lying, 48.9 +/- 5.4/h; sitting, 19.6 +/- 6.9/h; p less than 0.0005) and arterial oxyhemoglobulin saturation (Sao2) was increased (nREM; mean lying, 90.6 +/- 0.8%; mean sitting, 92.1 +/- 0.5%, p less than 0.005; minimum lying, 64.8 +/- 3.2%, minimum sitting, 80.8 +/- 2.1%, p less than 0.005). In approximately half the patients studied, obstructive sleep apnea was essentially abolished by the postural intervention. These patients were more obese and had lower Pao2 and higher Paco2 values awake than the remaining patients in whom the response was either incomplete or absent. Arousal from sleep was less frequent in the upright posture, but sleep efficiency and overall sleep architecture were unchanged. This simple maneuver may be useful for treating some patients with OSAS.