A blow to the head will impart rotational velocity to the brain and, depending on its magnitude, will produce effects ranging from concussion to profound neurological dysfunction. Resultant shear strains distort and rupture axons, blood vessels and major fibre tracts. Thirty-seven patients with head injury that was not complicated by significant hemorrhage or superficial laceration of the brain had coma or severe dementia, spastic quadriparesis, incontinence and autonomic dysfunction. These patients survived 24 hours to 243 days. Gross pathological examination revealed little, but there was microscopic evidence of axonal and small vessel injury in all; this was localized to the basal and midsagittal areas of the diencephalon and mesencephalon, particularly in those less severely injured. Such changes represent the basic pathology of all head injury. Data from this study suggest that concussion depends upon varying degrees of damage to the axon as well as the neuron. The current definition of concussion-immediate loss of consciousness with rapid and complete recovery of cerebral function-should not exclude the fact that a small number of neurons may have been permanently disconnected or have perished.