In order to assess the predictive value of certain biomechanical parameters for skull fracture after head injury, a prospective analysis was undertaken of a series of 5416 head-injured patients. In each case an assessment was made at presentation as to the velocity of impact and the physical properties of the impacting agent. The incidence of skull vault fracture was then calculated for injuries sustained at different velocities and for different types of contact. The incidence of fracture was also calculated with respect to the presence of post-traumatic amnesia (PTA). The incidences (95 per cent confidence intervals) of skull fracture at low, medium and high speeds were 0.17 (0.0427-0.433), 1.99 (1.47-2.63) and 10.2 (7.41-12.6) per cent respectively. Fractures tended to occur with greater frequency after impacts against broad hard surfaces or small objects rather than against broad soft objects. The percentage incidence of fracture in adults suffering more than 5 min of PTA was 17.93 (12.4-23.5) compared with 0.674 (0.372-1.1) in those without amnesia. Patient selection for skull radiography after an apparently minor head injury can be guided by an assessment of post-traumatic amnesia and the biomechanics of the injury.