Administration of continuous oxygen during ERCP may prevent hypoxia. Oxygen saturation was recorded using pulse oximetry in 50 consecutive patients undergoing ERCP. Patients were randomly allocated to receive no oxygen or low flow oxygen (2 liters/min) via nasal prongs or nasopharyngeal cannula. Oxygen saturation fell below 90% in 47% of patients not receiving oxygen compared with 0% in those administered oxygen (p less than 0.001). No difference existed in oxygen saturations between those groups receiving supplemental oxygen via nasal prongs or nasopharyngeal cannula. Continuous administration of low flow oxygen is recommended during ERCP.