The severity of injuries sustained by 2577 car occupants in road traffic accidents in the catchment area of one district accident service during February 1982 to January 1984 inclusive was assessed using the injury severity score system. In the first 12 months the mean injury severity score for front seat occupants injured in a road traffic accident was 4.94 and in the second 12 month period, after the implementation of the seat belt law, the mean injury severity score of all injured front seat occupants was 2.80. These figures indicated a reduction in injury severity of front seat occupants of 53.4% on the previous 12 month figures. The severity of injury sustained by unbelted front seat occupants and back seat passengers showed no significant change over the two years. The number of front seat occupants killed or sustaining serious injuries (injury severity score greater than 12) showed a reduction of 54% in the 12 months beginning February 1983. Front seat occupants requiring admission for injuries sustained showed a decline of 42% in the 12 months after the introduction of the seat belt law, and deaths among front seat occupants fell by 27% compared with the previous 12 months. After the implementation of seat belt legislation those front seat occupants killed or sustaining serious injuries included a significantly higher proportion of victims who were not wearing their seat belts or showed positive evidence of alcohol intake at the time of the accident. This series suggests that the incidence of serious injury or deaths among front seat occupants of cars has decreased substantially since the seat belt law became effective on 31 January 1983.