The effects of supplemental oxygen therapy on arterial oxygen saturation (Sao₂) in unsedated and sedated patients have been studied using pulse oximetry. Hypoxia (minimum Sao₂ 92% or less) occurred in 41% of unsedated patients breathing room air but in only 11% (p < 0.01) of those receiving supplemental oxygen (41/min) by nasal cannulae. Comparable effects were seen in sedated patients (60% versus 16%, respectively, p < 0.01). Hypoxia during upper alimentary tract endoscopy is common with or without sedation and can be reduced by supplemental oxygen.