An obligatory event in the pathogenesis of the multiple sclerosis plaque appears to be an increase in the permeability of the blood-brain barrier. Neuropathological observations of the brain of persons suffering from concussion after relatively minor head injury, as well as of animals subjected to experimental brain injury, have shown that alterations of the blood-brain barrier constitute a common result of such trauma. It is postulated that the alterations of the blood-brain barrier secondary to trauma of the brain or spinal cord of patients with already established multiple sclerosis may result in an exacerbation or recurrence of a previously symptomatic plaque, in the appearance of symptoms from a silent lesion, or in the formation of a new plaque in such an area of selected vulnerability. In other persons injury to the nervous system may cause the development of multiple sclerosis plaques in the previously damaged areas when the disease has its onset after the trauma. There is no evidence to support the idea that trauma ever causes multiple sclerosis.