Hypoxia as measured by pulse oximetry is well recognized in patients undergoing colonoscopy. A single trial of supplemental oxygen therapy has been shown to diminish the observed desaturation during colonoscopy. We further defined the role of supplemental oxygen during colonoscopy in a randomized controlled trial. Ninety-four patients undergoing routine colonoscopy were randomized to oxygen (2 L by nasal prongs) or placebo (nasal prongs only) arms. The physician and patients were blinded to which arm they were in. Sixty-four percent of patients in the placebo arm desaturated (