An experimental study in 39 ankles of the mechanisms of lateral ankle sprains was carried out using a simple qualitative apparatus. The mechanisms studied were: 1) supination-inversion, which was divided into ankle in plantar flexion and ankle in neutral position, since the orientation and tension of the ankle capsule and ligaments are related to the ankle position; 2) supination-internal rotation; and 3) supination-plantar flexion. Fresh amputation specimens were used. It was concluded that complete rupture of the anterior fibulotalar ligament is always the first lesion to occur in the lateral ankle sprain except when a supination force is applied to the ankle in neutral position when an incomplete tear of the fibulocalcaneal ligament may precede the total rupture of the anterior fibulotalar ligament. A partial rupture of the anterior deep fibers of the deltoid ligament can occur in extreme degrees of internal rotation or plantar flexion. A four-part grading system is suggested.