The risk of death of unbelted rear-seat occupants is obviously increased in car crashes. However, there is little epidemiological evidence that unbelted rear-seat occupants will also increase the risk of death of front-seat passengers. We compared risk of death and severe injury of front-seat occupants in car crashes with belted or unbelted rear-seat passengers. The risk of death of belted front-seat occupants with unbelted rear-seat passengers was raised nearly five-fold. If rear seatbelts had been used, almost 80% of deaths of belted front-seat occupants could have been avoided. Rear seatbelt use should be encouraged for the safety of all car occupants.