This two-year investigation was designed to estimate the incidence of driver neck pain in rear-struck vehicles involved in two-vehicle collisions and to determine the relationship between neck pain and specific vehicle, human, and environmental factors. Neck pain percentages were significantly higher for female (45%) than for male (28%) drivers. For female and male drivers, neck pain likelihood increased as head restraint height decreased below the head's center of gravity, although this effect was significant only for females. Head restraint backset, the horizontal distance measured from the back of the driver's head to the front of the head restraint, was not found to be related to neck pain for female drivers. Backset trends for male drivers could not be evaluated because few male drivers had head restraints that were high enough for backset to be relevant. Reported neck pain decreased for older drivers (females only), drivers in less severe crashes, and drivers in heavier cars (females only); all head restraint analyses were adjusted for these characteristics. Women, and most likely men, in the United States would benefit greatly from international harmonization to European head restraint standards. Until then, both women and men should be encouraged to adjust their adjustable head restraints, if possible, behind their heads' centers of gravity and to sit with the backs of their heads as close as possible to their head restraints.