Background and purpose
The effect of cognitive resources on the risk of dementia following traumatic brain injury (TBI) has hardly been investigated. The aim of this study was to examine the influence of cognitive ability and education in young adulthood on the association between TBI and dementia in men.

Method
A cohort of 658 447 Danish men, born between 1939 and 1959, who had been cognitively assessed at conscription were followed in the Danish National Patient Registry and the National Prescription Registry from 1977 through 2016 for incident TBI and dementia. The association between TBI and dementia was analysed using Cox proportional regression.

Results
During follow‐up, 29 781(4.5%) men experienced TBI and 10 971(1.7%) developed dementia. TBI was associated with a higher risk of subsequent dementia after adjustment for cognitive ability, education and psychiatric comorbidity. The risk estimate was higher for early‐onset dementia (hazard ratio 5.49, 95% confidence interval 4.97–6.06) than for dementia diagnosed after age 60 years (hazard ratio 2.85, 95% confidence interval 2.63–3.10). The association was slightly stronger in men with the highest cognitive scores or education than amongst those at lower levels.

Conclusion
Young adult cognitive ability did not explain a relatively strong association between TBI and dementia, and no evidence was found that cognitive ability or education was protective.