Diagonal seat belt application without accompanying lap belt closure may produce severe cervical spine injuries, including hangman's fracture and decapitation. Seat belts are effective in reducing injury, but they must be worn properly to do so. Passive restraint systems involving a diagonal seat belt may be hazardous if the motorist does not use the accompanying lap belt. We have presented a case in which the driver in a motor vehicle accident sustained a hangman's fracture (bilateral fracture of the pedicles of C-2) caused by use of a diagonal seat belt without accompanying lap belt closure. The mechanism of injury, as classically described in judicial hanging, is hyperextension and distraction, which occurred when the victim "submarined" under the diagonal seat belt and was caught at the neck.