This prospective study evaluated the incidence and severity of arterial oxygen desaturation during esophagogastroduodenoscopy (EGD) and assessed clinical factors in relation to arterial oxygen desaturation. Following pulmonary function testing, 192 patients underwent elective EGD with continuous recording of arterial oxygen saturation (SaO2). 62 patients (32.3%) showed arterial oxygen desaturation (SaO2 decrease greater than 4% from baseline SaO2); severe arterial oxygen desaturation (SaO2 less than or equal to 85%) reflecting hypoxemia (arterial oxygen tension less than or equal to 50 mmHg) was found in 17 patients (8.9%). These changes were most frequent at the esophageal stage of EGD but most marked at the gastroduodenal stage of EGD. The incidence of arterial oxygen desaturation was not related to age, sex, preendoscopic pulmonary function tests, smoking, or duration of endoscopy. These data support that continuous monitoring of SaO2 should be standard procedure during endoscopy because there was no identifiable preendoscopic risk factor for arterial oxygen desaturation during EGD. But there was no persistent drop in SaO2 longer than 2 minute. We suppose that oxygen supplement may not be required during EGD even in patients with modestly impaired pulmonary function tests.