A study of American football players' brains has found that 99% of professional NFL athletes tested had a disease associated with head injuries.
The report published on Tuesday in the Journal of the American Medical Association studied 202 deceased players - 111 of them from the NFL.
All but one former National Football League player were found to have chronic traumatic encephalopathy (CTE).
The study is the largest of its kind yet conducted, its authors say.
Dr Ann McKee, director of Boston University's CTE Center, which led the study, cautioned against drawing any immediate conclusions.
"There's a tremendous selection bias," she said, explaining how many of the brains were donated specifically by families who had suspected that their loved ones were suffering from CTE, which researchers believe is caused by repeated blows to the head.
"There's no question that there's a problem in football. That people who play football are at risk for this disease," she told US media.
The neurodegenerative brain disease CTE is typically found in people who have suffered repeated blows to the head, studies have found.
It causes a build-up of so-called tau proteins which can disable neuro-pathways and cause memory loss, impaired judgment, confusion and a variety of other mental health issues.
All 202 players studied - ranging in age from 23 to 89 - were required to have football as their primary exposure to head trauma.
Of the 202 total players, 87% were found to have traces of CTE.
It was also found in 48 of 53 college players and three of the 14 high school players.
Players featured in the study came from every position on the field, and from high school, university, and Canadian leagues, in addition to the NFL.
In a statement the NFL said it was grateful for the study and the "value it adds in the ongoing quest for a better understanding of CTE".
"There are still many unanswered questions relating to the cause, incidence and prevalence of long-term effects of head trauma such as CTE," the organisation continued.
"The NFL is committed to supporting scientific research into CTE and advancing progress in the prevention and treatment of head injuries."
The NFL in 2016 acknowledged for the first time that there is a connection between CTE and football.
In 2015, a federal judge approved a class-action lawsuit brought against the NFL by thousands of players, who had alleged they had suffered brain damage as a result of concussions.
Ron Sinai, of US legal funding firm Nova Legal Funding, said that similar lawsuits are being brought against the NCAA.
"College football players are having many of the same health issues as the pro players. In July 2016 a judge approved a $75 million settlement to resolve many, but not all, of these cases," he said.