BACKGROUND: Inversion ankle trauma is disabling, yet little is known regarding the incidence rate of first-time ankle sprains and how it is influenced by factors including sex, level of competition, and sport. HYPOTHESIS: The incidence rates of first-time ankle ligament sprains are influenced by sex, level of competition (high school vs college), and type of sports participation (basketball, soccer, lacrosse, and field hockey). STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: Between 1999 and 2003, high school and college athletes were evaluated before participation in their sports. Subjects were included in the study if they had not experienced a prior ankle or lower extremity injury and were then followed during participation in soccer, basketball, lacrosse, or field hockey to document their days of exposure to sport and injuries sustained. The relative risk associated with sex, level of competition, and sport was estimated by Cox regression. RESULTS: A total of 901 athletes had 50 680 person-days of exposure to sports, and 43 (4.8%) had an inversion injury that produced an ankle ligament sprain. Overall, the injury incidence rate was 0.85 sprains per 1000 person-days of exposure to sport. There were 0.68 and 0.97 ankle sprains per 1000 person-days of exposure to sport for the men and women, respectively. Although the risk of suffering an ankle sprain was higher for women than for men (relative risk, 1.51), the difference was not statistically significant (P = .21) and was owing to the increased risk in female basketball athletes compared to male basketball athletes (relative risk, 4.11; P = .045). Risk of injury was similar for the high school athletes in comparison to the college athletes (relative risk, 1.16). For the men, there was no difference in the risk of suffering an ankle sprain between the sports of basketball, soccer, and lacrosse, whereas for the women, the risk of suffering an ankle sprain was significantly greater during participation in basketball compared to lacrosse. CONCLUSION: In this study of first-time ankle sprains, for most sports, the incidence rate of inversion injury is less than 1 per 1000 days of exposure to sport, a value lower than previously reported. Among female athletes, ankle injury is associated with type of sport. Risk is highest for female basketball athletes, who are at significantly greater risk than male basketball athletes and female lacrosse athletes. The risk of first-time ankle injury is similar for high school and college-level athletes.