Prof Anna Nordstrom and colleagues from Umea University, Sweden report in the BMJ (doi: http://dx.doi.org/10.1136/bmj.f723) the results of a national prospective cohort study of male Swedish military conscripts which examined the association of mild traumatic brain injury and cognitive function in young men.They studied over 300,000 young men in military service from 1989-1994 and found in young men low cognitive function, intoxication, and factors related to low socioeconomic status were strong independent risk factors for mild traumatic brain injuries in men. The low cognitive function in twin pairs discordant for mild traumatic brain injury suggests a genetic component to the low cognitive function associated with such injuries. These results suggest that low cognitive function may be a risk factor for rather than the long term consequence of mild traumatic brain injury. This could be important to consider when evaluating cognitive function after mild traumatic brain injury, and perhaps especially after multiple such injuries. In athletes active in sports with a high risk of head injuries, the results also underscore the importance of pre-morbid cognitive testing for later comparison in those who sustain mild traumatic brain injuries.