IMPORTANCE: No consensus exists regarding the definition of high-risk surgery in older adults. An inclusive and precise definition of high-risk surgerymay be useful for surgeons, patients, researchers, and hospitals. OBJECTIVE: To develop a list of high-risk operations. DESIGN, SETTING, AND PARTICIPANTS: Retrospective cohort study and modified Delphi procedure. The setting included all Pennsylvania acute care hospitals (Pennsylvania Health Care Cost Containment Council [PHC4] April 1, 2001, to December 31, 2007) and a nationally representative sample of US acute care hospitals (Nationwide Inpatient Sample [NIS], Healthcare Cost and Utilization Project, Agency for Healthcare Research and Quality January 1, 2001, to December 31, 2006). Patients included were those 65 years and older admitted to PHC4 hospitals and those 18 years and older admitted to NIS hospitals.We identified International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) procedure codes associated with at least 1%inpatient mortality in the PHC4.We used a modified Delphi procedure with 5 board-certified surgeons to further refine this list by excluding nonoperative procedures and operations that were unlikely to be the proximate cause of mortality and were instead a marker of critical illness (eg, tracheostomy).We then cross-validated this list of ICD-9-CM codes in the NIS. MAIN OUTCOMES AND MEASURES: Modified Delphi procedure consensus of at least 4 of 5 panelists and proportion agreement in the NIS. RESULTS Among 4 739 522 admissions of patients 65 years and older in the PHC4, a total of 2 569 589 involved a procedure, encompassing 2853 unique procedures. Of 1130 procedures associated with a crude inpatient mortality of at least 1%, 264 achieved consensus as high-risk operations by the modified Delphi procedure. The observed inpatient mortality in the NIS was at least 1%for 227 of 264 procedures (86%) in patients 65 years and older. The pooled inpatient mortality for these identified high-risk procedures performed on patients 65 years and older was double the pooled inpatient mortality for correspondingly identified high-risk operations for patients younger than 65 years (6%vs 3%). CONCLUSIONS AND RELEVANCE: We developed a list of procedure codes to identify high-risk surgical procedures in claims data. This list of high-risk operations can be used to standardize the definition of high-risk surgery in quality and outcomes-based studies and to design targeted clinical interventions.