Background: There is a high risk for anterior cruciate ligament (ACL) injuries in alpine skiers. To reduce or try to prevent theseinjuries, intrinsic and extrinsic risk factors need to be identified.Purpose: To identify possible intrinsic and extrinsic ACL injury risk factors among competitive adolescent alpine skiers.Study Design: Case-control study; Level of evidence, 3.Methods: Between 2006 and 2009, a cohort of 339 alpine ski students (176 male, 163 female) from Swedish ski high schools wereprospectively observed in terms of ACL injuries. First-time ACL injuries were recorded. In September, prior to each ski season, theskiers were clinically examined according to a specific knee protocol.Results: Overall, 11 male and 14 female skiers sustained a total of 25 first-episode ACL injuries. The majority of injuries occurred inthe left knee (P < .05). Skiers who had participated in alpine skiing for >13 years (hazard ratio, 0.83; 95% CI, 0.68-1.00; P < .05) hada reduced risk of sustaining an ACL injury. Eighteen ACL injuries occurred during training, 12 in the technical discipline of giantslalom, and 8 in slalom. Fourteen skiers reported not to be fatigued at all at the time of injury, and 8 skiers reported that they weresomewhat fatigued.Conclusion: ACL injuries occurred more often in the left knee than the right. This should be taken into consideration in the design ofACL injury prevention programs. Those who reported a higher number of active years in alpine skiing showed a reduced risk ofsustaining an ACL injury. No other factor among those studied could be identified as an independent risk factor for ACL injury.