Medication errors in the hospital have been studied, quantified, and systematically evaluated for potential solutions. A notable exception is the perioperative setting, where medications given by anesthesiologists often bypass standard safety checks. This study is the largest prospective observational study of anesthesia-related medication events available to date. At least one medication error or adverse drug event occurred in nearly half of the 277 operations observed. Approximately 1 in 20 perioperative medication administrations resulted in a medication error or adverse drug event; 80% of these errors were deemed preventable. None of the errors resulted in death, but 2% were considered life-threatening. There were no differences in event rates among resident physicians, nurse anesthetists, and staff anesthesiologists. The study took place at an academic hospital with substantial local expertise in medication safety, where operating rooms already used a barcode-assisted syringe labeling system. An accompanying editorial suggests that medication error rates may therefore be even higher in other settings and community hospitals.