This study explores the relationships between the classification of traffic casualties by the police and by using the AIS. It examines a sample of accidents to car occupants that have been studied through the Cooperative Crash Injury Study data collection system in the United Kingdom. The results show that the police categories of ‘Slight’ and ‘Serious’ have poor correspondence with AIS rankings of the same individuals. In particular the ‘Serious’ police category includes a third of casualties who have either AIS 1or no injuries at all. The implications of these results on estimates of national casualties and costs are discussed. Some data from the European Union indicate major variations in definitions of casualties from country to country making national comparisons uncertain.