In September 2002, Bennet Omalu walked into the Allegheny County coroner’s office to find an unusual commotion. A Pittsburgh Steelers legend had died at 50, and his body was on the autopsy table. Omalu, a Nigerian-born neuropathologist who had only recently moved to Pittsburgh and knew almost nothing about American football, did not recognize the name on the chart. According to accounts of the autopsy, he asked the people around him, “Who is Mike Webster?” 

Webster had spent 17 years taking the hits that came with being one of the NFL’s most durable centers. By the end, his life had unraveled in ways that were difficult to explain. When Omalu opened Webster’s skull, there was nothing visibly damaged to explain what had happened to the 50-year-old former star. Omalu preserved the brain and took a closer look under the microscope. What he found would give football a name for a kind of damage it had not yet understood: chronic traumatic encephalopathy, or CTE.

More than two decades later, researchers are still trying to understand just how widespread that damage became among NFL players. A new study published in The British Medical Journal has now found CTE in 215 of 235 donated brains from former NFL players who died between 2016 and 2021, producing a minimum estimated prevalence of 24.5% across the full group, with the possible range extending as high as 97.7%. In their own words:

“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,” Daniel Daneshvar, an associate professor at Harvard Medical School, said. “These findings indicate that NFL players have a higher prevalence of CTE at death than has been previously shown in multiple community-based studies.”

Because CTE can only be confirmed through a brain dissection after death, much of what is known about its symptoms in life comes from the accounts of families and former players themselves. Many donors or their loved ones reported experiencing symptoms such as memory loss or major mood swings that led them to suspect the disease before death. The study also found a strong link between these lived symptoms and the most severe stage of the disease:

More than 90% of brains classified at Stage IV, the most advanced form of CTE, belonged to players who had also been diagnosed with dementia by a clinician based on medical records and interviews with loved ones. However, researchers caution that the picture is muddier at earlier stages, since roughly 40% of brains that showed no signs of CTE at all still came from players diagnosed with dementia in life.

Dementia, however, was not unique to CTE. Nine of the 23 donors without CTE had been diagnosed with dementia, while other brains showed conditions including Alzheimer’s and Lewy body disease. Overall, 202 of the 338 donors in the full study had dementia, and Stage IV CTE was associated with a 44% higher prevalence of dementia after adjustment for other factors.

The researchers also found that death certificates understated the problem. Of the donors diagnosed with dementia, only 30.7% had a primary cause of death linked to dementia or neurodegenerative disease. Even including secondary causes, the figure was just 40.6%.

The league is filled with examples of players who were diagnosed with CTE. Former NFL guard Conrad Dobler, who died in 2023 at the age of 72, was diagnosed with Stage 3 CTE. Another name on the list was the former NFL defensive back and former sports broadcaster Irv Cross, who suffered from chronic Stage 4 CTE. Cross died on February 28, 2021, at the age of 81. According to the researchers at Boston University, back in 2018, he was diagnosed with mild cognitive dementia.

But the new study was built differently from earlier brain-bank research. Researchers first identified all 878 former NFL players who died between 2016 and 2021, then compared that group with the 235 whose brains were examined. That matters because brain donors tended to have longer careers, more Pro Bowl appearances and greater prominence in the sport.

The 91.4% figure therefore applies only to the 235 brains examined. The 24.5% figure is the conservative minimum for all 878 players, assuming none of the 643 unexamined brains had CTE. The 97.7% figure is the opposite mathematical extreme, assuming all of them did, not an estimate that 97.7% actually had the disease.

“This isn’t looking at active players,” Daneshvar said. “This is people at the end of their lives.” He cautioned against extrapolating the findings to living or current NFL players.

“The bulk of the head impacts that these former NFL players experienced didn’t occur at the NFL level,” Daneshvar 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.”

That fits with earlier research linking repetitive head impacts to CTE, including impacts that do not cause a diagnosed concussion. A 2022 study led by Chris Nowinski and researchers from Oxford Brookes University and other institutions concluded that the evidence supported a causal relationship between repetitive head impacts and CTE.

“Sport governing bodies should not mislead the public on CTE causation while athletes die and families are destroyed by this terrible disease,” Nowinski said.

The league has also taken cognizance of the impacts of CTE on NFL players, and has acknowledged the link between repetitive trauma in football and CTE. NFL health and safety executive Jeff Miller was questioned by U.S. Representative Jan Schakowsky during a congressional committee roundtable discussion on concussions on March 14, 2016. During the questioning, Miller stated, “The answer to that question is certainly yes,” when the congressman asked him about the link between the game and neurodegenerative diseases.

“Football is a high-risk sport because of the routine hits, not just diagnosable concussions,” said Schakowsky. “What the American public needs now is honesty about the health risks and clearly more research.”

While questioning, Schakowsky cited the studies proposed by Ann McKee, a neuropathologist from Boston University. Those earlier brain-donation studies had found CTE in 90 of 94 NFL players, 45 of 55 college players and six of 26 high-school players examined. But those figures came from selected donors and could not establish prevalence across all players.

That selection problem is central to why the new study matters. Earlier UNITE Brain Bank research found CTE in 110 of the first 111 NFL players examined. The new analysis instead started with the known population of NFL deaths and used the donated brains to establish a possible range.

There is still no answer to why some heavily exposed players develop CTE while others do not. In the new study, lifetime football exposure was associated with more severe CTE, but not with whether CTE was present at all. Researchers said genetic and environmental factors may help explain that difference.

The study also largely reflects earlier generations of NFL players, so it cannot show whether newer helmets, practice restrictions, tackling techniques and concussion protocols have changed the risk for today’s players.

The NFL said it “continuously strives to make the game of football safer,” including through strategies to reduce concussions and head impacts, and urged former players concerned about their health to use its available resources.

For now, the clearest answer remains a postmortem one: at least nearly one in four former NFL players in this 2016–21 group had CTE at death. Whether that burden can be detected, or prevented, while players are still alive remains the larger question.