The application of epidemiologic methods in determining the nature and extent of athletic injuries occurring in high school football shows that (1) no significant difference in the rate of injury for players based on time exposed to risk exists, (2) injury risk increases with age, (3) a student with a history of prior football injury sustains injury at a significantly higher rate than his associates with no history, and (4) certain types of protective equipment are associated with greater injury incidence. These data are part of a five-year continuing study of athletes in 43 high schools.