A new study estimates that nearly three-quarters of former NFL players who died between 2008 and 2023 had chronic traumatic encephalopathy (CTE), a progressive brain disease associated with repeated head impacts in contact sports.
Published Wednesday in JAMA Network Open, the research was conducted by scientists affiliated with Boston University, Mass General Brigham and the Concussion & CTE Foundation. Researchers described the work as the first comprehensive effort to estimate CTE prevalence among deceased former NFL players.
The findings raise further questions about the long-term neurological risks associated with professional football. However, researchers emphasized that the estimate applies specifically to deceased former NFL players and should not be interpreted as the prevalence of CTE among current or living players.
Researchers Analyze More Than 2,300 Cases
The study examined 2,362 cases, combining information from 396 former NFL players who donated their brains for research with National Death Index records for 1,966 deceased players whose brains were not donated.
Nearly 94% of the donated brains showed evidence of CTE. Researchers then used the broader dataset to account for differences between players who donated their brains and those who did not, producing an estimated prevalence of approximately 75% among deceased former NFL players during the study period.
The researchers acknowledged that brain donors differed from non-donors in several important ways. Donors were more likely to be white, to have died at a younger age and to have spent more seasons in the NFL.
Those differences are important because previous CTE research has faced criticism over selection bias. Families may be more likely to donate a player's brain when the person experienced symptoms or other problems that raised concerns about brain health.
By incorporating records from players who did not donate their brains, the latest study sought to address some of those limitations.
“Even after accounting for differential brain donation, our findings suggest that the prevalence of CTE among deceased NFL players remains high,” the paper stated.
What the Findings Mean — and What They Don't
CTE is a neurodegenerative disease associated with repeated head impacts. It has been identified in athletes involved in contact sports, including American football.
The disease can currently be definitively diagnosed only after death through an examination of brain tissue. Researchers are continuing to investigate methods that could eventually allow doctors to identify CTE during a person's lifetime.
The study does not establish how many living NFL players have CTE. It also does not estimate prevalence among college athletes, high school players or children participating in youth football.
Another important limitation is that the findings do not establish whether the players identified as having CTE experienced symptoms during their lives or whether particular symptoms were caused by the disease.
CTE is classified into four stages, ranging from Stage I to Stage IV, based on the severity of the brain changes. Researchers noted that early-stage cases may produce few or no noticeable symptoms.
“Many cases may have been early stage at the time of death and possibly symptom free,” the paper said.
That distinction matters because the presence of CTE-related brain changes does not, by itself, reveal how a person functioned during life or whether the disease contributed to specific health problems.
Researchers Call for More Support and Better Diagnosis
Julia Gallini, the study's lead author, described the estimated prevalence as concerning and called for additional research and support for affected families.
She said greater efforts were needed “to support families, diagnose CTE during life, and treat CTE-related symptoms.”
Developing reliable diagnostic tools could help researchers better understand the disease, identify potential cases earlier and investigate possible treatments. For now, the inability to confirm CTE during life remains a major obstacle to studying its effects and determining its prevalence across different populations.
Researchers also highlighted the difficulties of extending the findings to younger football players. Limited information about deceased players who did not donate their brains makes it challenging to estimate prevalence among athletes who played at the high school or college levels.
Consequently, the study does not answer whether the risks observed among former NFL players translate directly to athletes who participated in football for fewer years or at lower levels of competition.
Earlier Research Produced a Different Estimate
The new findings follow an earlier study published in August that estimated approximately one in four former NFL players who died between 2016 and 2021 had CTE.
The two estimates cover different periods and research approaches, making direct comparisons difficult without considering the underlying methods and assumptions.
The latest study's broader timeframe and inclusion of non-donor records were intended to provide a more comprehensive estimate. Nevertheless, the authors acknowledged that uncertainties remain, particularly because the available data cannot fully eliminate every potential source of bias.
The results add to a growing body of research examining the relationship between repeated head impacts and long-term brain health in football.
NFL and Players Association Respond
The NFL said in a statement that it continues to pursue measures designed to reduce the risks associated with playing football.
“The NFL continuously strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts,” the league said.
It also encouraged former players to use available resources to assess their health concerns and seek appropriate treatment.
The NFL Players Association said studies of this kind are important for understanding the potential long-term consequences of playing football.
Such research may help inform future efforts to improve player safety, develop diagnostic tools and provide support for former athletes experiencing health problems.
A Significant Finding With Important Limitations
The estimated 75% prevalence underscores the importance of continued research into CTE and the potential consequences of repeated head impacts in professional football.
But the number needs to be interpreted in context. It describes an estimated proportion of deceased former NFL players, not all former players, current athletes or people who have played football at any level.
Researchers still need better ways to diagnose CTE during life, understand its relationship with symptoms and establish how the risks differ across levels of football participation.
For players, families and medical professionals, those unanswered questions remain central to understanding the long-term effects of the sport and improving care for athletes after their playing careers end.
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