Since the degenerative brain disease known as C. T. E. was first scientifically linked to football about 20 years ago, and as hundreds of former professional football players have been diagnosed with the disease after their deaths, a haunting question has lingered: Just how many N.

F. L. players will end up with C. T.

E.? New data provides a sobering indication: At least one in four of all the people who have played in the N. F. L. might expect to end up with chronic traumatic encephalopathy, according to a study of hundreds of cases over a recent six-year period.

Researchers considered every former N. F. L. player who died from 2016 to 2021.

There were 878. Some died in their 20s, some in their 80s, most in between. The study’s main finding was simple math: At least 215 of those 878 former players — 24.5 percent — had C.

T. E. The actual prevalence could be far higher.

The roughly 25 percent rate does not consider the 643 brains that were not examined. Some of those, maybe many of them, likely had C. T.

E. too. C. T.

E. is a progressive neurological disease caused by repeated impacts to the head. It can be definitively diagnosed only by examining the brain after death.