Elderly patients undergoing endoscopic retrograde cholangiopancreatography (ERCP) have an increased risk of sedation related complications during the procedure. To determine whether nasal oxygen supplementation (2 l/min) reduces these risks, half of 66 patients aged over 60 undergoing ERCP using minimal midazolam sedation alone were randomised to receive nasal oxygen. The arterial oxygen saturation and pulse rate of all patients were monitored by pulse oximetry before and during the procedure. Only three patients in the oxygen supplemented group (n = 33) required any form of intervention for hypoxia compared with six in the control group (n = 33). Comparison of mean arterial oxygen saturation between the groups showed significantly higher levels in the nasal oxygen group throughout the procedure. Pulse rate comparisons showed no significant difference from control group values, both groups had short periods of significant tachycardia. We conclude that minimal sedation with midazolam alone still produces hypoxia during ERCP in a substantial number of elderly patients. Nasal oxygen supplementation increases the level of patient oxygenation and reduces the need for intervention, but does not reduce tachycardia in the elderly patient. Because hyoscine may be a significant factor contributing to the tachycardia, sparing rather than routine use of this agent is advisable.