All events prolonging an endoscopic procedure or recovery, or requiring a medication or an intervention, were analyzed from a consecutive sample of 508 patients receiving conscious sedation. Although 102 events were identified (20%), 33 of these (7%) were major. These included four episodes of apnea and four patients with a prompt and sustained fall in oxygenation during the procedure; 19 additional patients had a decrease to less than 89% in oxygen saturation in the 30 minutes after the completion of the procedure. The patients with observed events had significantly more major illnesses, a higher fraction older than 70 years, and a higher fraction of endoscopic retrograde cholangiopancreatography than those without events (p < 0.05) but had a similar dose of sedative medications, mean age, and fraction of colonoscopies.