OBJECTIVE Conscious sedation increases the tolerance of patients to endoscopic examination, but arterial oxygen desaturation during endoscopy has often been reported. Elderly patients are considered to be at special risk. However, sedation-induced changes in cardiovascular function in the elderly have not been well investigated. Therefore, this study examined the effects of sedation on pulse rate, systolic blood pressure, and arterial oxygen saturation in elderly patients. METHODS Twenty patients aged less than 60 years and 20 patients aged 60 years or more, scheduled to undergo colonoscopic removal of colorectal polyps, received 0.05 or 0.025 mg/kg body weight, respectively, of midazolam intravenously. Before, during, and after endoscopy, pulse rate, systolic blood pressure, and arterial oxygen saturation were recorded every 5 min with a pulse oximeter. RESULTS Administration of midazolam had no significant effect on pulse and arterial systolic blood pressure in either group. In contrast, midazolam significantly reduced the levels of arterial oxygen saturation during and after endoscopy in both age groups. However, although patients aged 60 years or more received midazolam at half dosage, arterial oxygen saturation during endoscopy was significantly lower and the incidence of 5% or more oxygen desaturation was significantly higher than in the patients aged less than 60 years. CONCLUSIONS Midazolam increases the tolerance of elderly patients to endoscopy, but it significantly lowers the level of arterial oxygen saturation and increases the incidence of desaturation. Continuous monitoring is especially valuable during endoscopy in the elderly.