To determine whether oxygen desaturation and cardiac arrhythmias occur in children during esophagogastroduodenoscopy with the use of conscious sedation, we prospectively studied 34 consecutive patients between the ages of 2 months and 18 years. Patients with pulmonary, cardiac, and neurologic disorders were defined as high risk and those without were defined as normal. All patients received intravenous sedation with meperidine, diazepam, or midazolam, used alone or in combination. Pulse oximetry, respiratory rate, and lead II electrocardiogram were recorded throughout all episodes of desaturation and tachycardia. Oxygen desaturation to less than 90% occurred in 68% of normal patients and in 58% of high-risk patients during esophagogastroduodenoscopy. Seventy-five percent of the high-risk patients and 82% of the normal patients had an arrhythmia during esophagogastroduodenoscopy usually associated with oxygen desaturation. Sinus tachycardia was the most common arrhythmia, although other arrhythmias were identified. Despite the frequency of oxygen desaturation and cardiac arrhythmias, no adverse outcome was observed in any patient. Most episodes of oxygen desaturation and cardiac arrhythmia resolved spontaneously. Subdivision of patients into high-risk groups by age, sex, weight, or diameter of endoscope used did not allow prediction of oxygen desaturation or cardiac arrhythmia. Our data suggest that conscious sedation in children undergoing esophagogastroduodenoscopy is safe and free of significant adverse clinical problems. However, conscious sedation during esophagogastroduodenoscopy continues to have certain inherent risks. Therefore we strongly advocate the routine use of continuous cardiac rhythm and pulse oximetry monitoring of all children during esophagogastroduodenoscopy performed with the use of conscious sedation.