Fueling fresh fears about the health risks in America's most popular sport, a new study of deceased NFL players found 96 percent of those tested suffered from brain disease.
The survey for PBS television's Frontline program found that out of 91 NFL players tested, 87 showed signs of the degenerative disease known as chronic traumatic encephalopathy (CTE).
The study was carried out by researchers for the Department of Veterans Affairs and Boston University.
A wider sample of players from all levels of American football, who played either in high school, college, semi-professionally or professionally, found CTE in the brain tissue of 131 out of 165, or 79 percent.
CTE is widely thought to be caused by repetitive trauma to the head and can lead to conditions such as nausea, memory loss and dementia.
The report cautioned that the results came with certain caveats, most notably the fact that CTE could only be definitively tested for posthumously.
Most of the brains studied were from players who had donated their brains for testing because they suspected they were suffering from CTE.
Nevertheless, Ann McKee, the director and chief of neuropathology at the Veterans Affairs Boston Healthcare System, said the results were consistent with previous research indicating a link between football and brain injury.
"People think that we are blowing this out of proportion, that this is a very rare disease and that we are sensationalizing it," McKee said.
"My response is that where I sit, this is a very real disease. We have had no problem identifying it in hundreds of players."
A spokesman for the NFL said the league was "dedicated to making football safer and continue to take steps to protect players, including rule changes, advanced sideline technology, and expanded medical resources."
The NFL had donated $1 million to Boston's brain bank in 2010. The league has also taken steps aimed at reducing head injuries, with concussions falling by 35 percent since 2012.
In April the NFL agreed to settle a lawsuit and pay $765 million to about 5,000 former players over health claims.
However some players have already made headlines this year by choosing to walk away from multi-million-dollar contracts citing health concerns.
The issue is also likely to come into the spotlight again with a Hollywood movie -- "Concussion" -- in the works starring Will Smith.
The film is about forensic pathologist, Dr. Bennet Omalu, one of the first to diagnose degenerative brain disease in former NFL players.