Background: Pediatric sedation for diagnostic radiological procedures remains the mainstay for adequate imaging quality. Objectives: To clarify the risk of adverse events during anesthesiologist-led sedation or anesthesia for diagnostic radiological procedures in children in order to improve quality of care. Methods: We enrolled children aged <15 years given sedation or anesthesia by an anesthesiologist and scheduled for computed tomography, magnetic resonance imaging, or nuclear medicine imaging November 2010-September 2014. We recorded adverse events occurring in the first 24 h. Results: Of 1,042 patients enrolled, adverse events were recorded in 254 (24.4%, 95% confidence interval [CI] 21.9 to 27.1). Adverse respiratory events occurred in 31 (3.0%), cardiovascular events in 7 (0.7%), sedation was prolonged in 165 (15.8%), there was one case of contrast allergy (0.01%), and there were 50 other minor complications (4.9%). Of the respiratory complications, there were 14 of airway obstruction (1.3%), 2 of apnea (0.2%), 14 of oxygen desaturation (1.3%), and one of laryngospasm (0.01%). There were no life threatening complications or consequences. Age <1 year (adjusted odds ratio [adjusted OR] 2.5, 95% CI 1.2 to 5.3) and American Society of Anesthesiologists (ASA) physical status classification 2 and 3 (adjusted OR 4.6, 95% CI 1.1 to 19.8, and adjusted OR 6.3, 95% CI 1.3 to 30.9, respectively) were risk factors for respiratory complications. Conclusions: Adverse events were common during sedation or anesthesia, but no life threatening or sentinel events occurred under experienced supervision. Caution should be exercised in children <1 year or with an ASA classification >1.