Forty-five cases of diffuse axonal injury (DAI) brought about by nonmissile head injury in humans are analyzed and compared with 132 cases of fatal head injury without DAI. All cases were subjected to a comprehensive neuropathological study. In the patients with IIAI a statistically significant lower incidence of lucid interval, fracture of the skull, cerebral contusions, intracranial hematoma, and evidence of high intracranial pressure were found, with a higher incidence of head injury due to road traffic accident. Brain swelling and hypoxic brain damage were not statistically different in the two groups. The features of DAI in humans are compared with the DAI that has been produced in subhuman primates by pure inertial loading brought about by angular acceleration of the head. The available evidence indicates that DAI in human beings occurs at the time of head injury and is not due to complicating factors such as hypoxia, brain swelling, or raised intracranial pressure.