OBJECTIVE: The research was undertaken to describe the injury severity score (ISS) and the new injury severity score (NISS) and to illustrate their statistical properties. DESIGN: Descriptive analysis and assessment of the distribution of these scales. METHODS: Three data sources--the National Pediatric Trauma Registry; the Massachusetts Uniform Hospital Discharge Data Set; and a trauma registry from an urban level I trauma center in Massachusetts--were used to describe the distribution of the ISS and NISS among injured patients. RESULTS: The ISS/NISS was found to have a positively skewed distribution and transformation did not improve their skewness. CONCLUSION: The findings suggest that for statistical or analytical purposes the ISS/ NISS should not be considered a continuous variable, particularly if ISS/NISS is treated as a continuous variable for correlation with an outcome measure.