As a consequence of various federal and provincial initiatives to promote the use of seat belts in Canada, the wearing rate of seat belts among front outboard passenger car occupants is now estimated at 90 percent. Accordingly, the vast majority of air bag deployments in Canada involve restrained occupants. In order to gain a better understanding of the field performance of air bag systems, Transport Canada recently initiated an m-depth study of motor vehicle collisions involving air bag deployments. To date, investigations have been completed on 242 such collisions. While the preliminary data suggest that supplementary air bag systems provide considerable added protection against serious head injuries in moderate and high severity frontal crashes, they also suggest that, in low severity crashes, deployment of an air bag system may expose belted occupants to unnecessary injury risk from the air bag itself. A more favourable trade-off can be achieved by increasing the deployment threshold of the air bag system for belted occupants.