A volunteer woman subject incurred injury to her right knee consisting of a torn anterior cruciate ligament and stretched medial collateral ligament during a lateral (+Gy) impact test. Similar injury has not been reported in the English-language literature an accidental sideward automotive crashes or lateral impact experimentation involving humans. The primary mechanism which produced this injury was external tibial rotation on the femur with the knee flexed. The factors contributing to the injury included extraordinarily forceful leg bracing by the subject, her knee joint laxity or hypermobility, and the absence of side supports to limit lower extremity flailing during the impact response. In future lateral impact tests, women subjects should be used with caution and any subject with abnormal joint mobility should be excluded from participation.