Background: This study focused on whether there are clear indications of seat belt use to be found at autopsy, evaluating the sensitivity and specificity of seat belt marks (SBM), and whether use of seat belt and seating location affects the type and severity of injuries sustained. Methods: Information on the type of injuries sustained and seatbelt use was retrieved from autopsy reports and police reports respectively, for cases of fatal motor vehicle collisions occurring in Sydney, Australia over a 5 year period. Results: A SBM was only found on restrained occupants. The proportion of restrained occupants with evidence of a SBM was 36% (sensitivity), whilst unrestrained occupants showed no evidence of a SBM (100% specificity). A SBM was also found to reliably reflect seating position of occupant. Conclusion: Restrained occupants can be expected to show evidence of the seat belt in just over 1/3 of cases. A spurious SBM is very unlikely to be present if the occupant was unrestrained.