Severe head injury in early adulthood may increase the risk of dementia in older age, but it is not known whether head injury in later life also increases the risk of dementia. A representative sample (82%) of persons aged 70 years or older with a Mini-Mental State Examination (MMSE) test score of > or =26 (n = 325) were followed-up for 9 years to record all their fall-related head injuries resulting in traumatic brain injury (TBI). At the end of the follow-up period, 152 persons (81% of the surviving population) were examined for clinical dementia, according to DSM-IV criteria. Eight persons sustained a TBI and 34 developed dementia. Brain injury was associated with younger age at detection of dementia even when adjusted for sex and educational status (low educational status significantly associated with dementia); age-specific hazard ratio (95% confidence interval) 2.80 (1.35-5.81). In a population scoring > or =28 points in the baseline MMSE an apolipoprotein E (ApoE) epsilon4 phenotype was also associated with younger age at the time of detecting dementia; 3.56 (1.35-9.34), and the effect of brain injury and ApoE epsilon4 phenotype was synergistic; 7.68 (2.32-25.3). We conclude that fall-related TBI predicts earlier onset of dementia and the effect is especially high amongst subjects who carry the ApoE epsilon4 allele.