CONTEXT: Ankle injuries are the most common sport-related injuries. To date, no studies have been published that use national data to present a cross-sport, cross-sex analysis of ankle injuries among US high school athletes. OBJECTIVE: To investigate the incidence rates of ankle injuries by sex, type of exposure, and sport. DESIGN: Descriptive epidemiologic study. SETTING: One hundred US high schools. PATIENTS OR OTHER PARTICIPANTS: United States high school athletes. MAIN OUTCOME MEASURE(S): We reviewed ankle injury data collected over the 2005-2006 school year from a nationally representative sample obtained by High School RIO, an injury surveillance system. Specific sports studied were boys' football, boys' and girls' soccer, girls' volleyball, boys' and girls' basketball, boys' wrestling, boys' baseball, and girls' softball. RESULTS: An estimated 326 396 ankle injuries occurred nationally in 2005-2006, yielding an injury rate of 5.23 ankle injuries per 10 000 athlete-exposures. Ankle injuries occurred at a significantly higher rate during competition (9.35 per 10 000 athlete-exposures) than during practice (3.63) (risk ratio = 2.58; 95% confidence interval = 2.26, 2.94; P < .001). Boys' basketball had the highest rate of ankle injury (7.74 per 10 000 athlete-exposures), followed by girls' basketball (6.93) and boys' football (6.52). In all sports except girls' volleyball, rates of ankle injury were higher in competition than in practice. Overall, most ankle injuries were diagnosed as ligament sprains with incomplete tears (83.4%). Ankle injuries most commonly caused athletes to miss less than 7 days of activity (51.7%), followed by 7 to 21 days of activity loss (33.9%) and more than 22 days of activity loss (10.5%). CONCLUSIONS: Sports that combine jumping in close proximity to other players and swift changes of direction while running are most often associated with ankle injuries. Future research on ankle injuries is needed to drive the development and implementation of more effective preventive interventions.