Injury severity scales of proven reliability and validity are essential for the appropriate allocation of therapeutic resources, for prediction of outcome, and for evaluation of the quantity and quality of emergency medical care in differing facilities and over time. Quantitation of injury severity in the field is particularly necessary. Existing scales are too imprecise to permit comparisons of management or systems of care. In this paper, the authors present the Triage Index, a measure of injury severity based on five simple variables observed in a design data set of 1084 patients. The Triage Index has been developed with state-of-the-art multivariate statistical techniques, meets the requirements of an interval ranking scale, and has been both validated and assessed for interuser reliability. The Triage Index is proposed as a validated system of early, rapid, noninvasive, accurate patient assessment permitting appropriate matching of trauma victims with available therapeutic resources as a means of reducing mortality and morbidity.