The mechanism of injury and the signs and symptoms associated with initial injury to the anterior cruciate ligament were investigated in 23 subjects. The subjects had injured only their anterior cruciate ligament with no other clinically definable laxity to other major ligamentous structures. The results showed that 70% of subjects had injured their anterior cruciate ligament at footstrike during noncontact situations. In 53% of these subjects the mechanism of injury was internal rotation of the tibia with the knee in slight flexion. A shifting sensation occurring between the tibia and femur was noted by 61% of subjects, while 30% recalled a crack or popping sound at the time of injury. Only 17% of subjects could continue in sports activity immediately following the injury, all with difficulty.