Six high school football players, nine college football players, seven pro boxers and four N.H.L. players, including Derek Boogaard, the former hockey enforcer who died from an accidental overdose of alcohol and painkillers, also showed signs of C.T.E. The study also included 21 veterans, most of whom were also athletes, who showed signs of C.T.E.

The study was conducted by investigators at the Boston University Center for the Study of Traumatic Encephalopathy and the Veterans Affairs Boston Healthcare System, in collaboration with the Sports Legacy Institute. It took four years to complete, included subjects 17 to 98 years old, and more than doubled the number of documented cases of C.T.E. The investigators also created a four-tiered system to classify degrees of C.T.E., hoping it would help doctors treat patients.

The volume of cases in the study “allows us to see the disease at all stages of severity and how it starts and spreads in the brain, which gives us an idea of the mechanism of the injury,” said Ann McKee, the main author of the study, who is a professor of neurology and pathology at Boston University School of Medicine and works at the V.A. Boston.

Those categorized as having Stage 1 of the disease had headaches and loss of attention and concentration, while those with Stage 2 also had depression, explosive behavior and short-term memory loss. Those with Stage 3 of C.T.E., including Duerson, a former All-Pro defensive back for the Chicago Bears who killed himself last year, had cognitive impairment and trouble with executive functions like planning and organizing. Those with Stage 4 had dementia, difficulty finding words and aggression.

Despite the breadth of the findings, the study, like others before it, did not prove definitively that head injuries sustained on the field caused C.T.E. To do that, doctors would need to identify the disease in living patients by using imaging equipment, blood tests or other techniques. Researchers have not been able to determine why some athletes who performed in the same conditions did not develop C.T.E.