For the first time, there's now evidence linking head knocks in rugby league to brain injury.
Researchers from a Sydney hospital and university are behind the study, which involved examining the brains of two former long-term players.
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Two former NRL players, now deceased, were diagnosed with CTE, or chronic traumatic encephalopathy.
"We'd look at 100 or brains per year, and these two brains stood out," Associate Professor Michael Buckland told 7 News.
CTE causes memory loss, confusion, depression and can lead to Alzheimer's. Problems can arise years after repeated knocks to the head.
"The brain its natural state is like firm gelatin," Buckland told 7 News. "It's soft and easily torn.
"With knocks, we know that the brain is shaking about inside the skull, tearing some nerve fibres."
The NRL says it will review these findings, and that significantly increase investment into better understanding and managing head injuries in contact sports.
"It's a lot safer now than it was back in the day," Cronulla Sharks fullback Matt Moylan said.
NRL great Peter Stirling is one of more than 80 athletes who have pledged to donate their brains, with researchers hoping to find a way to stop CTE in its tracks.