The pulse oximeter is a widely used noninvasive monitor which provides a continuous reading of arterial oxygen saturation. We sought to determine if patients with cardiopulmonary disease were at higher risk of oxygen unsaturation than controls during gastrointestinal endoscopy. Patients presenting for upper gastrointestinal endoscopy and/or colonoscopy were placed into one of two groups: A. control population:patients with no history of cardiac and/or pulmonary disease (N = 11); B. test population:patients with a history of cardiac and/or pulmonary disease (N = 39). The procedure was performed by one of 14 equally experienced physicians. The Nellcor N-100 Pulse Oximeter and D-25 finger probe were used to monitor arterial oxygen saturation which was recorded at five minute intervals from commencement of intravenous sedation analgesia to 15 minutes post-procedure. In both groups, the oxygen saturation fell to statistically significant. A: 98.3% to 90.2%; B: 97.7% to 89.5%. However, the amount of unsaturation which occurred in Group B was not statistically different from Group A. Therefore, history of prior cardiac and/or pulmonary disease does not increase the degree of unsaturation from that occurring in patients with no such history. In addition, the effects of age, procedure, duration, I.V. sedation, and individual tolerance on oxygen unsaturation were studied by regression analysis and analysis of variance (ANOVA). Only the upper GI endoscopy patients showed a dependence of oxygen unsaturation upon one of the variables, specifically patient tolerance. It is concluded that patients at risk for hypoxemia during endoscopy or colonoscopy cannot be determined prior to the initiation of the procedure, and it is recommended that all of these patients be given supplemental oxygen as well as monitored with a pulse oximeter.