Study objective: To develop a new trauma decision rule. Design: Retrospective clinical review. Setting: Level 1 trauma center. Type of participants: 1,004 injured adults. Measurements and main results: A new trauma decision rule was derived from 1,004 injured adult patients using a new operational definition of major trauma. The rule, termed the Trauma Triage Rule, defines a major trauma victim as any injured adult patient whose systolic blood pressure is less than 85 mm Hg; whose motor component of the Glasgow Coma Score is less than 5; or who has sustained penetrating trauma of the head, neck, or trunk. Using the operational definition of major trauma, the rule had a sensitivity of 92% and a specificity of 92% when tested on the 1,004-patient cohort. Conclusion: The Trauma Triage Rule may significantly reduce overtriage while only minimally increasing undertriage. This approach must be validated prospectively before it can be used in the prehospital setting.