Figures from the nation's largest brain bank show that 96 percent of the deceased NFL players tested over the past decade had positive results for a degenerative brain disease connected with concussions.
The research, done by the Department of Veterans Affairs and Boston University and published by Frontline, identified chronic traumatic encephalopathy in 87 of 91 NFL players. CTE also appeared in 79 percent (131 of 165) of all football players studied.
An ESPN The Magazine/Outside the Lines report in August also revealed the 96 percent rate of CTE in NFL players.
"People think that we're blowing this out of proportion, that this is a very rare disease and that we're sensationalizing it," Dr. Ann McKee, who runs the lab as part of a collaboration between the VA and BU, told Frontline. "My response is that where I sit, this is a very real disease. We have had no problem identifying it in hundreds of players."
Researchers have said CTE is triggered by repeated head trauma and can cause memory loss, depression and dementia.
Former NFL tight end Tom Crabtree responded to the findings Friday on Twitter.
What an exciting way to start my weekend. I probably have CTE. https://t.co/1Ct0MK9qCT— Tom Crabtree (@itsCrab) September 18, 2015
The NFL said in a statement that it is "dedicated to making football safer and continue to take steps to protect players, including rule changes, advanced sideline technology, and expanded medical resources."
"We continue to make significant investments in independent research through our gifts to Boston University, the [National Institutes of Health] and other efforts to accelerate the science and understanding of these issues," the league added.
The NFL gave a $1 million research grant to the brain bank in 2010.
The 96 percent rate found in the study is consistent with a smaller sample size announced last year by the Department of Veterans Affairs in Massachusetts.
CTE can be definitively diagnosed only after death, though brain scans are being used to try to identify signs of the disease in living players. The deceased players used in the study had agreed to donate their brains for testing.
Information from ESPN's Kevin Seifert contributed to this report.