No large-scale epidemiologic study to investigate how sex and age affect fatal trauma risk from the same impact has been possible previously because large numbers of people are rarely subject to sufficiently similar major physical insults. This paper describes such a study, made possible by two recent developments: first, the availability of a large data file, the Fatal Accident Reporting System, which gives information on more than one third of a million people fatally injured in traffic crashes; second, a new technique, the double-pair comparison method, which, by focusing on vehicles containing two occupants, at least one of whom is killed, allows appropriate inferences to be made from such data. Fatality risk versus sex and age was determined for ten categories of vehicle occupants (unbelted car drivers, helmeted motorcycle passengers, etc.). Similar effects were found for different occupants, indicating that basic physiologic response (not confined to traffic fatalities) is being measured. It is found that fatality risk is about 25% greater for females than for similar aged males from about age 15 to 45 years. At younger and older ages males are more at risk. For both sexes, fatality risk is least at age 20 years. At age 70 it is about three times what it is at age 20.