There is increasing evidence of a link between head injury and the subsequent onset of Alzheimer's disease. Deposits of amyloid beta-protein (A beta) are found not only in cases of dementia pugilistica but in some 30% of patients dying after a single episode of severe head injury. Detailed clinicopathological studies have shown that A beta deposition is most likely, but not exclusively, to occur, the older the patient at the time of injury, and if the injury is the result of a fall. Distribution studies have shown that the A beta is widely deposited in the neocortex and there is no apparent association with any of the multiple primary or secondary pathologies of traumatic brain injury. There is an increased expression of beta-APP particularly in the pre-alpha cells of the entorhinal cortex and in areas of axonal damage. Recent molecular genetic studies have shown that there is a strong association between deposits of A beta and the apolipoprotein E genotype of the individual.