Research on 202 former American football players has found evidence of a brain disease in nearly all of them, from NFL athlete level down to high school.
Chronic traumatic encephalopathy (CTE) has been linked with repeated head blows and the results confirm that it can happen even in young players.
The report only reflects a high occurrence in samples at a Boston brain bank, and many donors contributed because of troubling symptoms before their death.
CTE was diagnosed in 177 former players, or nearly 90% of the brains studied.
Researchers still do not know how common the disease is in American football or the general population. Some players with repeated concussions never develop it.