American football has the highest documented rate of CTE (chronic traumatic encephalopathy) of any sport. In the largest study of its kind, researchers at Boston University examined the brains of 202 deceased football players and found CTE in 110 of 111 former NFL players, a rate of 99 percent. No other sport comes close to that figure in available research, though combat sports like boxing rank second.
These numbers come with an important caveat: brain bank studies rely on donated brains, often from players whose families suspected something was wrong. That inflates the percentages. Still, the sheer volume of confirmed cases in football, combined with data from other contact sports, paints a clear picture of where the greatest risk lies.
American Football: The Highest Documented Risk
The 2017 Boston University study, published in the Journal of the American Medical Association, remains the most cited evidence. Of the 202 brains examined, 177 showed signs of CTE. The most striking finding was at the professional level: 110 out of 111 former NFL players had the disease. But CTE wasn’t limited to pros. The study also found it in college and even high school players, though at lower rates.
The risk scales with time spent playing. Research from Spaulding Rehabilitation found that every additional year of playing American football is associated with a 15 percent increase in the odds of being diagnosed with CTE. Among those who already had the disease, each extra year raised the odds of a severe diagnosis by 14 percent. This means a player with a 20-year career spanning youth leagues through the NFL faces dramatically higher risk than someone who played only in high school.
Starting age matters too. A Boston University study published in Brain Communications found that players who began tackle football at a younger age had measurably lower levels of key brain proteins decades later, a sign of structural damage to the brain’s white matter. Researchers involved in the study support delaying tackle football until age 14, recommending flag football for younger children.
Boxing and Combat Sports
Boxing has been linked to brain damage for over a century. The condition was originally called “dementia pugilistica,” or punch-drunk syndrome, before researchers recognized it as part of the broader CTE spectrum. A study from the Cleveland Clinic’s Professional Athletes Brain Health Study, involving 130 active and retired professional fighters in boxing, martial arts, and mixed martial arts, found that 40 percent met the criteria for traumatic encephalopathy syndrome, a cluster of cognitive and behavioral symptoms considered a likely precursor to CTE. Over 80 percent of those affected reported boxing as their only form of fighting.
The challenge with combat sports data is that CTE can only be definitively diagnosed after death through autopsy. Traumatic encephalopathy syndrome is the closest clinical proxy available in living athletes, and while it strongly correlates with CTE, it’s not the same as a confirmed diagnosis. Still, the combination of frequent, high-force blows to the head makes boxing one of the most dangerous sports for long-term brain health.
Rugby and Ice Hockey
Rugby carries measurable neurological risk, though confirmed CTE cases are fewer than in football. A study published in the Journal of Neurology, Neurosurgery & Psychiatry tracked former international rugby union players and found they were 2.67 times more likely to be diagnosed with a neurodegenerative disease compared to the general population. About 11.4 percent of former international players received such a diagnosis during the study period, versus 5.4 percent of the comparison group. Multiple autopsy-confirmed CTE cases have been reported in former rugby players.
Ice hockey has produced several high-profile CTE cases, including former NHL enforcers whose roles involved frequent fighting. However, large-scale prevalence studies comparable to the football brain bank data don’t yet exist for hockey, making it difficult to assign a reliable percentage.
Soccer: A Lower but Real Risk
Soccer sits in a different category. CTE has been confirmed in fewer than 10 soccer players through autopsy, all of whom played at the semi-professional or professional level. The primary concern is heading the ball. While a single header doesn’t deliver the force of a football tackle, professional players may head the ball thousands of times over a career. Those repeated, low-level impacts accumulate over decades.
Some leagues have responded by restricting heading in youth soccer. The science here is still developing, but the confirmed cases suggest that even a sport without violent collisions can produce CTE when repetitive head contact is part of the game.
Why Repetitive Hits Matter More Than Concussions
One of the most important findings in CTE research challenges a common assumption. The CDC states there is no strong evidence that a single concussion or even occasional head impacts lead to CTE. Instead, the disease is associated with long-term exposure to repeated hits to the head. This means the hundreds of subconcussive impacts a lineman absorbs every season, none of which produce obvious symptoms, are likely more dangerous than the occasional big hit that results in a diagnosed concussion.
This distinction explains why football linemen, who collide on every snap but rarely get concussions, show high rates of CTE. It also explains why sports with constant low-level head contact, like boxing sparring or soccer heading, carry risk even when dramatic knockouts are rare.
Why CTE Is Still Hard to Measure
A major limitation in all CTE research is that the disease can only be confirmed through brain autopsy. There is currently no blood test, brain scan, or clinical exam that can definitively diagnose CTE in a living person. Boston University received NIH funding in 2025 to study potential biomarkers using blood draws and specialized brain imaging, but that work is still in progress.
This means every prevalence number available today comes from brain bank studies, where families chose to donate tissue, usually because the athlete showed symptoms before death. The true rate of CTE across all players in any sport is almost certainly lower than what brain bank studies report, but researchers don’t yet have the tools to measure it in the broader population. What the data does show clearly is the ranking: football and boxing at the top, rugby and hockey in the middle tier, and soccer with the fewest confirmed cases but growing concern.