Oxygen saturation and pulse rate were recorded using pulse oximetry during 50 consecutive endoscopic retrograde cholangiopancreatography (ERCP) examinations. Oxygen saturation dropped to below 90% in 22 patients at some point during the procedure. Multiple regression analysis revealed that increasing age and weight were the most significant variables in predicting tachycardia or hypoxia. These problems cannot be reliably predicted or detected clinically. The use of a pulse oximeter during ERCP is recommended.