OBJECTIVE: To evaluate the effect of supplementary oxygen on heart rate and arterial oxygen saturation during colonoscopy. DESIGN: Controlled study. SETTING: Two university hospitals, Denmark. SUBJECTS: 40 patients having colonoscopy. INTERVENTIONS: 20 patients were given supplementary oxygen through nasal prongs (2 L/min), and 20 patients breathed room air during colonoscopy. All patients were given conscious sedation and were monitored with a pulse oximeter during colonoscopy. MAIN OUTCOME MEASURES: Tachycardia (pulse rate>100 min(-1)) and arterial oxygen desaturation (SpO2