Study finds CTE in at least 25 percent of former NFL players who died 2016-2021

A separate recent analysis detected the disease in 99% of former NFL players who had donated their brains for research. However, a significant limitation of that study was in its potential bias: It was done in a group of people whose brains had been donated by family members who may have noticed symptoms while the person was still alive.

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The researchers compared data from death certificates against the number of brain donors with confirmed CTE. They further narrowed the time period to between 2016 and 2021, when the highest number of brain donations had been received.

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Of 878 former players who died in that time frame, 235 had donated their brains for research. Among those, 215 had been diagnosed with CTE. Many of the donations had been analyzed at brain banks at Boston University and the University of California, San Francisco.

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Dr. Rachel Grashow, director of research initiatives for the Football Players Health Study at Harvard University, said the new study is "important," but she also noted that a diagnosis of CTE didn't always correlate with symptoms. CTE is diagnosed in four stages based on location and accumulation of the protein tau.

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The NFL's 2011 Collective Bargaining Agreement limited the number of contact practices during the season. The current CBA was ratified in 2020 and runs through March 2031. The Ivy League banned full-contact tackling during practices five years later.

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To better understand what proportion of all NFL players that sample represents, the study's authors created a list of every player who had appeared in at least one NFL game since 1949. They referenced demographic data and obtained death certificates for all deceased players from the Centers for Disease Control and Prevention to create a denominator list of 1,712 players who died in the same time period. The researchers then narrowed the sample to those who had died between 2016 and 2021, when the proportion of players whose brains were donated for study was highest.

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To better understand what proportion of all NFL players that sample represents, the study's authors created a list of every player who had appeared in at least one NFL game since 1949. They referenced demographic data and obtained death certificates for all deceased players from the Centers for Disease Control and Prevention to create a denominator list of 1,712 players who died in the same time period. The researchers then narrowed the sample to those who had died between 2016 and 2021, when the proportion of players whose brains were donated for study was highest.

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The prevalence estimate of 25% is at the "low end," Daneshvar said. The actual figure could be higher, the paper noted, pointing to a number of CTE diagnoses made outside of the brain bank sample, along with other suspected cases of CTE in former football players whose brains were not evaluated after death. Additionally, researchers wrote, an analysis of death certificates of football players who did not donate their brains revealed another 215 whose certificates listed a neurodegenerative disease as a cause of death.

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But the link was far less clear between dementia and lower stages of CTE, or even those whose brains had been found to be free of the disease's characteristic buildups of tau protein. Some 40% of CTE-free samples were diagnosed with dementia.

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Degenerative brain diseases can affect anyone, even those without a history of brain injury. Among Americans over the age of 55, the lifetime risk of eventually developing any form of dementia is 42%, one study published last year in the journal Nature Medicine found.

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The 25% figure represents a conservative floor, researchers said, and the true prevalence of CTE at death was probably higher. Because CTE can only be diagnosed after death, the research did not reveal how common the disease is among active players or living retirees.

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Still, Daneshvar cautioned against applying that number to living players. "This isn't looking at active players," he said. "This is people at the end of their lives. And so we can't directly extrapolate from this to living current players or even living retired players."

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The association between the most advanced CTE and dementia is important, Grashow said, but the study still cannot predict the health or future of an individual living player. Her research on living former football players is funded in part by the NFL Players Association, which she said has no authority over the research or its publication. She does not receive NFL funding.

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More than 60% of the players with CTE were determined to have had dementia, based on family interviews and medical records. Among the 104 players with Stage 4 CTE, more than 90% had dementia. After adjusting for age and other factors, researchers found that Stage 4 CTE was associated with a 44% higher prevalence of dementia.

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More than 60% of the players with CTE were determined to have had dementia, based on family interviews and medical records. Among the 104 players with Stage 4 CTE, more than 90% had dementia. After adjusting for age and other factors, researchers found that Stage 4 CTE was associated with a 44% higher prevalence of dementia.

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The study's authors acknowledged several limitations. Brain donors tended to have played longer, achieved greater prominence in the sport and died younger than the other former players. Their families also were more likely to have observed cognitive, behavioral or mood symptoms. Clinical histories were reconstructed after death through family interviews and medical records, making them susceptible to imperfect recall.

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The study was conducted by researchers from Mass General Brigham, Boston University and other institutions. Ann McKee, director of Boston University's CTE Center, was a co-author. Most of the examined brains came through Boston University's UNITE Brain Bank, with additional cases from Mount Sinai and the University of California at San Francisco.

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The study also largely reflects players from earlier eras of professional football. It cannot determine whether more recent changes in equipment, practice rules, tackling instruction and concussion management have altered the risk faced by players today.

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The actual prevalence could be far higher. The roughly 25% rate does not consider the 643 brains that were not examined. Some of those, maybe many of them, likely had CTE too.

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For now, most brains of NFL players are never examined, so an unknown number of possible CTE cases go undiagnosed. As awareness of the disease grows, family members are increasingly donating the brains of recently deceased former players who displayed CTE's stew of symptoms. The brains that are donated are not a random sample. Telltale signs of the disease were often evident.

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If the new study's statistics were applied to today's players — 1,696 on the league's 32 regular-season rosters, not including more than 500 members of practice squads and those ineligible due to injury — at least 400 of them would be diagnosed with CTE.

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Studies have shown that the rate of CTE in people who have not sustained repeated impacts to the head is nearly zero.

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For years, the NFL downplayed the risks of repeated head trauma, often insisting that not enough is known about CTE to draw conclusions about its dangers, origins and prevalence. Uncertainty has left room for inaction; scientists have yet to decipher, among many things, why some people can play football for decades and not get CTE, while others have had severe cases in their teens.

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Making such an estimation is complicated by unknowns. What is certain, researchers said, is that CTE is preventable.

"We know what's causing it," said Dr. Daniel Daneshvar, the lead author of the study, the chief of brain injury rehabilitation at Spaulding Rehabilitation Hospital and an associate professor at Harvard Medical School. "It's a neurodegenerative disease where we know and have complete control over the risk. And understanding the extent of that risk is one of the steps into making sure that we can mitigate and decrease that risk."

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Others become notorious because of criminal behavior that might later be explained, in part, by CTE, like Aaron Hernandez and Phillip Adams, who committed murder and were diagnosed with CTE after their own deaths. In July 2025, Shane Tamura, who played football but did not reach the NFL, killed four people in a New York City building that houses the league's headquarters. After Tamura's death, an autopsy confirmed that he had CTE.

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At least two of his former offensive linemates, both guards, were diagnosed with it: Tom McHale, who died at 45; and John Bruhin, who died at 57. Another teammate, linebacker Keith McCants, had CTE and died at 53.

If the new data were applied to the rest of his teammates over his career, Grimes is likely to have played with dozens of CTE victims, who either died without being tested or who are aging quietly in the shadows.

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Others become notorious because of criminal behavior that might later be explained, in part, by CTE, like Aaron Hernandez and Phillip Adams, who committed murder and were diagnosed with CTE after their own deaths. In July 2025, Shane Tamura, who played football but did not reach the NFL, killed four people in a New York City building that houses the league's headquarters. After Tamura's death, an autopsy confirmed that he had CTE.

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Researchers tallied all of the former NFL players known to have died between 2008 and 2021 (2,079 of them) and found that 16.6% — 1 in 6 — had been diagnosed with CTE.

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The growing roster of CTE cases has done little to diminish football's popularity. The NFL reportedly earned more than $23 billion in revenue in 2024.

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Only a few cases reach public consciousness, usually when attached to a famous name, like Junior Seau, Ken Stabler and Frank Gifford, all inducted into the Pro Football Hall of Fame.

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In 2017, 110 of the first 111 brains of former NFL players that had been donated to that point were diagnosed with CTE.

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Scientists hope to soon be able to clinically diagnose CTE in the living with a large degree of certainty, the way that other neurodegenerative diseases, like Parkinson's and Alzheimer's, often are.

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The growing sample size of CTE cases makes for stark realizations. For example, eight members of the 1972 Miami Dolphins, the only undefeated team in modern NFL history, have been diagnosed with CTE.

The latest, previously unreported, was Mercury Morris, the speedy halfback who played eight seasons in the NFL, struggled with headaches and drug addiction in retirement, and died in 2024 at age 77.

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Not all CTE cases belong to long-retired players in their geriatric years. The 2001 New England Patriots, who won the Super Bowl, had seven former players die between the ages of 35 and 50, according to The Boston Globe. At least three were diagnosed with CTE, researchers said.

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They narrowed the data to the most recent six-year period because, they said, it coincided with a surge of brain donations after several well-publicized cases of CTE and the 2015 film "Concussion." (In 2014, 17 brains were donated to what is now called the UNITE Brain Bank. In 2018, 53 were.)

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The 2009 Chargers, a team that went 13-3 in the regular season, already have three deceased players who were found to have CTE and whose names were made public.

One was Vincent Jackson, a three-time Pro Bowl receiver who died in a hotel room of alcohol poisoning at 38. Another was Kevin Ellison, a defensive back who died at 31 when he was struck by a car after wandering onto a California freeway. The other was defensive back Paul Oliver, who was 29 in 2013 when he shot himself on the stairs of his family home in front of his wife, Chelsea, and their two young sons.

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Her husband played four years in the NFL and sustained a series of concussions, she said, and then became someone "who just wasn't him." He lived with headaches, became delusional and paranoid, and began drinking heavily, she said. The couple discussed the prospect of CTE shortly after Seau, the longtime Chargers star who finished his career with New England, shot himself in the chest to preserve his brain for testing.

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The latest research, however, suggests that at least 10 others from that team might have CTE, if the 1-in-4 ratio applies.

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