The European Commission requires its Member States to count serious road traffic casualties as ?victims with at least an AIS 3 or superior injury score? (AIS3+). Most European countries use hospital data for this purpose. However, injuries are coded according to ICD, which does not assign a severity score. A conversion tool is therefore necessary (ICD-AIS map, American Association for Automotive Medicine) to obtain injury severity. As classifications' structures are different, ICD conversion leads to an important number of undetermined AIS scores. The map converts ICD-10-CM (Clinical Modification - US coding) to AIS 2005 update 2008, but some countries use different ICD-10 modifications. In France, a road trauma registry includes all road accident casualties occurring in the Rhône county. Every injury is directly coded according to AIS 2005 (update 2008) by an expert physician. Hospital data (called PMSI in France), a national medico- administrative database, provides ICD-10 codes for all hospitalised victims. A probabilistic linkage between the registry and hospital data on the Rhône county will be performed. Around 9000 hospitalised victims are reported in the registry for 2015- 2021 and these should also be in the hospital discharge data. We hence expect a large linked sample. This linked database will provide, for injury ICD codes, the observed AIS codes (RTSN and AIS severity score), and their distribution. This will be very useful for ICD codes without a corresponding AIS3+ status in the existing mapping tool. This will also enable calculation of a concordance score between directly-coded AIS severity scores and converted ones. Furthermore, the registry AIS expert is working on a one-to-several mapping between ICD-10 and AIS (RTSN) codes. Conversion improvements could thus be proposed for French data, and probably for European ones as well. Extension to other AIS severity scores or study of sequelae (FCI) could also be possible.