OBJECTIVE: To analyze risk factors, including postural sway, ankle strength, and flexibility, for the prediction of ankle injuries in men's high school basketball players. DESIGN: A cohort study with follow-up duration of 1 basketball season. SETTING: Biomechanics laboratory. PARTICIPANTS: Forty-two (age, 16.5+/-1.1y) players competing in first league of the High School Basketball Association without history of injury in the lower extremities within 6 months before recruitment and without significant malalignment in the lower extremities were included. None of these players met exclusion criteria such as using ankle braces or taping or failed in wearing low-top sports shoes during the follow-up season. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Biomechanic measurements including isokinetic ankle strength, 1-leg standing postural sway, and ankle joint dorsiflexion flexibility were performed before the basketball season by 1 physical therapist. The subsequent monthly follow-up questionnaires were sent and returned by mail to prospectively record the incidence of ankle injury occurring in the season. Results of these preseason measurements were analyzed to correlate if any of these measured variables could predict future ankle injuries. RESULTS: Eighteen ankle sport injuries were recorded for 42 players during the follow-up season. High variation of postural sway in both anteroposterior and mediolateral directions corresponded to occurrences of ankle injuries (P=.01, odds ratio [OR]=1.220; P< .001, OR=1.216, respectively). All other variables were not associated with injury. CONCLUSIONS: High variations of postural sway in 1-leg standing test could explain partly the increased prevalence of ankle injury in basketball players. It may be used as a screening tool to recommend balance training before basketball season.