The influence of oxygen supplementation on the prevalence of hypoxemia during endoscopic procedures was studied in 289 patients in a prospective clinical trial. The frequency of oxygen desaturation was found to decrease significantly in patients receiving oxygen via nasal prongs, compared to patients not receiving oxygen supplementation. The effect was demonstrated especially in patients who underwent an additional procedure during their endoscopic examinations. Similar effects were found in patients undergoing gastroscopy, colonoscopy, and endoscopic retrograde cholangiopancreatography. In view of the risks related to hypoxemia and its high prevalence in endoscopic procedures (28-50%) we recommend that a routine oxygen supplementation policy be considered in every patient undergoing endoscopy, especially when additional procedures are to be performed.