Background:​ Among all high school sports, football has the highest rate of injury. Prior research has been limited primarily because of challenges in surveillance, defining injuries, and measuring exposures.

Hypothesis:​ Football injury patterns differ across player and session characteristics.

Study Design:​ Descriptive epidemiology study.

Methods:​ More than 5000 football players from 87 high schools in California were observed for 2 seasons (2001 and 2002). School representatives were trained to collect data on injuries, player characteristics, and daily exposures. Data were analyzed using descriptive statistics and clustered Poisson regression.

Results:​ Players sustained 25.5 injuries per 100 players, 9.3 injuries per 10 000 player-hours, and 8.4 injuries per 100 session-hours. Session rates were highest during games, on artificial turf (13.8 of 100), during foggy weather (25.1 of 100), and on clear evenings (21 of 100). Offensive and defensive backfielders had about a 20% increased rate of injury compared with linemen. The adjusted injury rate for starters was 60% higher than the rate for nonstarters (relative rate, 1.6;​ 95% confidence interval, 1.4–1.9).

Conclusion:​ Risk profiles differed by experience, playing position, and surface types. We recommend future sports injury research that measures time-dependent exposures at the individual level and for various types of environmental playing conditions.