In the largest study of its kind, researchers have found that traumatic brain injury is associated with an increased risk of dementia. The risk of dementia was highest among people who had suffered multiple TBIs. But even a single mild TBI was tied to an increased risk of dementia.

TBI has a wide range of severity. It extends from a mild sports concussion — an elbow to the head in a basketball game, for example — that results in brief or no unconsciousness and no structural harm to the brain to the most severe brain injuries that can cause extended unconsciousness, coma or even be fatal.

The study, in Lancet Psychiatry, used Danish health databases that included all residents as of Jan. 1, 1995, who were at least age 50 at some time during the 36-year follow-up, from 1977 to 2013. Among 2,794,852 people, they found 132,093 who had had at least one TBI.

After adjusting for medical, neurological and psychiatric illnesses, they found that compared with people who had never had a TBI, those who had had any were at a 24 percent increased risk for dementia, and those who had had five or more had nearly triple the risk. Even a single mild TBI increased the risk by 17 percent. For their first TBI diagnosis, 85 percent were this mild type.

Still, the lead author, Dr. Jesse R. Fann, a psychiatry professor at the University of Washington School of Medicine, said the absolute risk of getting dementia as young as 50 was quite low. “I don’t want people to think that just because you’ve had a head injury you’re bound to get dementia.”