Hypothesis: We hypothesized that wrong-site surgery is infrequent and that a substantial proportion of such incidents are not preventable by current site-verification protocols. Design: Caseseriesandsurvey of site-verification protocols. Setting: Hospitals and a malpractice liability insurer. Patients and Other Participants: All wrong-site surgery cases reported to a large malpractice insurer between 1985 and 2004. Main Outcome Measures: Incidence, characteristics, and causes of wrong-site surgery and characteristics of site-verification protocols. Results: Among 2 826 367 operations at insured institutions during the study period, 25 nonspine wrong-site operations were identified, producing an incidence of 1 in 112 994 operations (95% confidence interval, 1 in 76 336 to 1 in 174 825). Medical records were available for review in 13 cases. Among reviewed claims, patient injury was permanent-significant in 1, temporary-major in 2, and temporary-minor or temporary-insignificant in 10. Under optimal conditions, the Joint Commission on Accreditation of Healthcare Organizations Universal Protocol might have prevented 8 (62%) of 13 cases. Hospital protocol design varied significantly. The protocols mandated 2 to 4 personnel to perform 12 separate operative-site checks on average (range, 5-20). Five protocols required site marking in cases that involved nonmidline organs or structures; 6 required it in all cases. Conclusions: Wrong-site surgery is unacceptable but exceedingly rare, and major injury from wrong-site surgery is even rarer. Current site-verification protocols could have prevented only two thirds of the examined cases. Many protocols involve considerable complexity without clear added benefit.