338 donated NFL players' brains are giving researchers an unusual window into CTE and chronic brain injury, raising fresh questions about the long-term cost of football

For a football player, a collision can last less than a second. The consequences, however, may become visible decades later, when the roar of the stadium has disappeared and the athlete is long retired.

That possibility is at the centre of a new study published in the BMJ in August 2026, which examined deaths among former NFL players and found an extraordinarily wide range of possible chronic traumatic encephalopathy (CTE) prevalence. The researchers identified CTE in 315 of 338 former NFL players whose brains had been donated for examination. Depending on how the researchers account for the players who did not donate their brains, the estimated prevalence among players who died between 2008 and 2021 ranged from 18.5% to 98.7%.

The numbers are striking, but the story is more complicated than saying almost every NFL player develops CTE.

What is CTE?

Chronic traumatic encephalopathy, or CTE, is a neurodegenerative disease associated with repeated head impacts, including blows that may not cause a diagnosed concussion. It is marked by abnormal deposits of a protein called tau in specific areas of the brain.

Researchers have linked advanced CTE with problems including memory and thinking difficulties, behavioural changes and dementia, although the symptoms and severity can vary. CTE can only be definitively diagnosed after death by examining brain tissue for these characteristic changes.

What the new study actually found

Researchers led by Daniel H. Daneshvar and colleagues in 2026 examined a cohort of 1,712 former NFL players who died between 2008 and 2021. Of those, 338 had donated their brains to research.

Pathologists found CTE in 315 of those 338 brains, or 93.2%. CTE was also frequently severe: 104 donors had stage IV disease, the most advanced category used in the study.

But only about one in five of all the deceased players in the study had donated a brain. That creates a fundamental problem. Researchers know what was found in the donated brains, but they cannot directly examine the brains of the remaining 1,374 players. So the study calculated a range rather than pretending to know the exact prevalence.

Under its most conservative assumption, the 315 confirmed cases represent the only CTE cases among all 1,712 deaths, producing an estimate of 18.5%. At the other extreme, if every player whose brain was not examined also had CTE, the figure would reach 98.7%.

Why brain donation makes the numbers complicated

The researchers acknowledge that people who donate their brains may not represent the average former NFL player.

For example, donors in the study tended to have characteristics associated with longer or more prominent football careers, including more years playing and more Pro Bowl appearances. Families may also be more likely to donate when an athlete experienced neurological problems.

That means the 93.2% figure among donors should not be treated as the prevalence of CTE among all NFL players.

A response published in the BMJ after the study also highlighted another important limitation: most NFL players are still alive, meaning the study's prevalence estimates concern players who died during the specified period, not the current NFL population or every player who has ever competed.

A 2023 Nature Communications study by Daniel H. Daneshvar and colleagues examined 631 donated brains from football players. Researchers found CTE in 451, or about 71%, and found that cumulative exposure to head impacts, particularly estimated linear and rotational acceleration, was associated with CTE pathology. Players with higher-stage CTE had, on average, longer football careers and substantially greater estimated cumulative head-impact exposure.