A new study from researchers at Boston University estimates that roughly 75% of deceased NFL players had at least some degree of chronic traumatic encephalopathy, or CTE, the degenerative brain disease linked to repeated head impacts rather than a single injury.
CTE can currently only be diagnosed after death. The Mayo Clinic describes it as a progressive condition tied to repeated blows to the head over time, identified in four stages, with stage 4 representing the most severe form.
The research examined more than 2,300 former NFL players who were 80 or older and died between 2008 and 2023. Of that larger group, nearly 400 had donated their brains for study. Among those 396 donors, 93.5% were diagnosed with CTE.
Researchers cautioned that the donor group likely skews toward more severe cases. They estimated that former players with clear cognitive symptoms while alive were roughly five times more likely to donate their brains than those without such symptoms. Donors also died younger on average, at about 67 years old, compared with nearly 75 for the broader group studied.
Because many donors may have died with only early-stage CTE, the study noted that some of them could have been symptom-free at the time of death. “Many cases may have been early stage at the time of death and possibly symptom free,” the researchers wrote.
The study also found that an estimated 27.2% of the players had a history of all-cause dementia during their lives, though the researchers did not conclude that CTE was the cause of that dementia.
A Growing Body of Research
This study follows an earlier report released in August that found at least 1 in 4 people who have played in the NFL might expect to develop CTE at some point in their lives.
Symptoms associated with CTE include memory loss, mood swings and erratic behavior. The disease was first identified in boxers but has become closely associated with football due to the repeated head impacts players absorb on nearly every play. It has also been documented in athletes from hockey, rugby and other contact sports.
CTE’s connection to football entered public consciousness after prominent players, including Junior Seau, Mike Webster and Frank Gifford, were diagnosed with the disease after their deaths. Webster, examined by Dr. Bennet Omalu, became the first NFL player diagnosed with CTE, a finding that helped trigger the league’s ongoing reckoning with concussions.
That reckoning remains current: the NFL recorded 168 documented in-game concussions during the 2025 season, a 30% increase from the 129 recorded in 2024. The league also logged a total of 538 evaluations and 26 medical timeouts related to head injuries during the season.
