To investigate the association between traumatic brain injuries (TBIs) and the risk of young onset dementia (YOD), that is, dementia before 65 years of age.

The study cohort comprised 811,622 Swedish men (mean age = 18 years) conscripted for military service between 1969 and 1986. TBIs, dementia, and covariates were extracted from national registers. Time‐dependent exposures using Cox proportional hazard regression models were evaluated.

During a median follow‐up period of 33 years, there were 45,249 men with at least 1 TBI in the cohort. After adjustment for covariates, 1 mild TBI (hazard ratio [HR] = 1.0, 95% confidence interval [CI] = 0.5–2.0), at least 2 mild TBIs (HR = 2.5, 95% CI = 0.8–8.1), or 1 severe TBI (HR = 0.7, 95% CI = 0.1–5.2) were not associated with Alzheimer dementia (AD). Other types of dementia were strongly associated with the risk of 1 mild TBI (HR = 3.8, 95% CI = 2.8–5.2), at least 2 mild TBIs (HR = 10.4, 95% CI = 6.3–17.2), and 1 severe TBI (HR = 11.4, 95% CI = 7.4–17.5) in age‐adjusted analysis. However, these associations were largely attenuated after adjustment for covariates (1 mild TBI: HR = 1.7; at least 2 mild TBIs: HR = 1.7; 1 severe TBI: HR = 2.6; p < 0.05 for all).

In the present study, we found strong associations between YOD of non‐AD forms and TBIs of different severity. These associations were, however, markedly attenuated after multivariate adjustment. Ann Neurol 2014;75:374–381