OBJECTIVES: To compare sport and gender differences in injury rates and proportions of injuries related to illegal activity and to describe the epidemiology of injuries related to illegal activity. DESIGN: Descriptive epidemiology study. SETTING: 100 US high schools. SUBJECTS: Athletes participating in nine sports: boys' football, soccer, basketball, wrestling, and baseball plus girls' soccer, volleyball, basketball, and softball. MAIN OUTCOME MEASURES: Illegal activity-related injuries were analyzed using data from the 2005-06 and 2006-07 National High School Sports-Related Injury Surveillance Study. RESULTS: Nationally, an estimated 98 066 injuries were directly related to an action that was ruled illegal activity by a referee/official or disciplinary committee, giving an injury rate of 0.24 injuries per 1000 athletic competition-exposures. Boys' and girls' soccer had the highest rates of injuries related to illegal activity, and girls' volleyball, girls' softball, and boys' baseball had the lowest. Overall, 6.4% of all high school sports-related injuries were related to illegal activity, with the highest proportion in girls' basketball (14.0%), girls' soccer (11.9%), and boys' soccer (11.4%). A greater proportion of injuries related to illegal activity were to the head/face (32.3%) and were concussions (25.4%) than injuries not related to illegal activity (13.8% (injury proportion ratio 2.35; 95% CI 1.82 to 3.04; p<0.001) and 10.9% (injury proportion ratio 2.35; 95% CI 1.71 to 3.22; p<0.001), respectively). CONCLUSIONS: Illegal activity is an overlooked risk factor for sports-related injury. Reducing illegal activity through enhanced enforcement of sports' rules and targeted education about the dangers of illegal activity for players, coaches, and referees/officials may reduce sports-related injuries.