A new study might make parents think twice about football.

Research on 200 former players finds evidence of brain disease in nearly all of them, around 99 percent.

The number includes athletes in the NFL, all the way down to high school.

It's the biggest update on what's known as CTE, caused by repeated hits to the head.

But there is a flaw in the study. 

Researchers only tested people who may have shown symptoms of CTE before death.