For years, as researchers have established a link between repeated impacts to the head and the degenerative brain disease known as CTE, one major question has loomed: Just how common is it — especially among high-risk groups, like professional football players?

A new study from researchers at Mass General Brigham, published Tuesday in the prominent medical journal The BMJ, seeks to establish a more accurate baseline of CTE prevalence among retired NFL players who have died. Of football players who appeared in at least one NFL game in their careers and who died between 2016 and 2021, the study found that a minimum of around 25% had CTE upon their death.

"We've known that this disease process can be caused by, in part, repeated traumatic brain injury. But we didn't know how common it was," said Dr. Daniel Daneshvar, the study's lead author.