The NACA-scale is used in many Austrian, German and Swiss emergency medical systems for demographic description of emergency patients. Little attention has been payed to the evaluation of its construct and predictive validity. In 427 consecutive trauma patients rescued in primary mission the NACA-Scale and the Injury Severity Score (ISS) were determined. Outcome data were obtained from medical charts and by written or telephone requests. Data were analysed with Spearman-Rank-Correlation. NACA-Grade and ISS-values showed only a moderate correlation with a considerably large spread (Rho = 0.721). Both severity scores demonstrated a good correlation to mortality (Rho = 0.976/0.994) and to transfer to an ICU (Rho = 0.964/0.943), as well a moderate correlation to the duration of ICU-stay (Rho = 0.722/0.756) and of hospital stay (Rho = 0.558/0.694). The NACA-scale adequately describes life threat in trauma victims and correlates well with morbidity and mortality. Thus, it is a valuable tool for demographic purposes in emergency medical systems. For more precise prehospital severity rating in trauma patients, the NACA-scale should be supplemented or replaced by a physiologically based prehospital severity score.