The patient was not named in the study, but study lead author Dr. Bennet Omalu confirmed that it was former NFL player Fred McNeill, who died in 2015, CNN reported.
Until now, the only way to diagnose CTE was after death. McNeill's diagnosis was made in 2012 using an experimental brain scan that can detect a signature protein of CTE called tau, according to the study in the journal Neurosurgery.
While widely associated with former football players, CTE can develop in anyone who suffers repeated head injuries, including soldiers, wrestlers and boxers.
The experimental brain scanning technology has been used on at least a dozen other former NFL players but McNeil's case is the first to have the CTE test results confirmed with an autopsy, CNN reported.
Omalu and his colleagues are now seeking funding to begin a phase 3 clinical trail of their technology. He said it could be less than five years before a commercial test is available.