As many as 97.7% of NFL players who died between 2016 and 2021 could have had brain disease related to repeated blows to the head, a new study says.
Researchers looked at 878 former players who died during that period, with 235 having donated their brains to research.
Of those, 215 were found to have chronic traumatic encephalopathy (CTE), a degenerative brain disease which can only be diagnosed after death.
The study, published in the British Medical Journal, external, also found dementia was common among brain donors.
"We and others have shown that in NFL players, specifically, there is about a four times higher rate of death from neurodegenerative disease than the general population," said the report's lead author Daniel Daneshvar, an associate professor at Harvard Medical School.
"These findings indicate that NFL players have a higher prevalence of CTE at death than has been previously shown in multiple community based studies."
Researchers calculated a "conservative minimum" of CTE prevalence among those who died in the six-year period to be 24.5% - rising to a "maximum" 97.7%.
They have urged the NFL to take prevention measures to reduce repetitive head impacts across all levels of the sport.
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"The bulk of the head impacts that these former NFL players experienced didn't occur at the NFL level," Danshvar said.
"They occurred at the collegiate level and at the high school level and in many cases the youth level. And all of those cumulative hits to the head added up to result in an increased risk."
An NFL spokesperson said: "The NFL continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts.
"The NFL remains committed to ensuring that the NFL community has access to a robust - and expanding - set of resources to enhance their physical and mental well being.
"We encourage former players to utilise these resources to identify and seek treatment when they are concerned about their health."
CTE among American football players has been a growing issue in recent years. A 2011 Collective Bargaining Agreement for the NFL limited the number of contact practices.
Following a long-running class action, in 2016 the NFL agreed a $1bn (£700m) settlement with more than 5,000 former players.
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