This study compared the therapeutic efficacy between cast immobilization and functional treatment of grade III ruptures of the lateral ankle ligaments. Subjects ( n=121) had closed physeal cartilage, age under 35 years, grade III rupture without previous or associated injuries, and practiced regular sports. Patients were randomized into an immobilization group (21 days plaster cast) or a functional one (15 days strapping plus early controlled mobilization). Symptoms (pain, swelling, stiffness, subjective instability), joint laxity, return to preinjury activity (time and level) and rate of reinjury were assessed 3, 6, and 12 months after sprain. Objective joint laxity was related to constitutional laxity, creating a new variable [talar tilt at injury - talar tilt at control]/contralateral talar tilt. The functional group showed significantly earlier and better return to physical activity, fewer symptoms at 3 and 6 months but no intergroup difference at 12 months. Functional treatment also showed better decrease in joint laxity. No intergroup differences were found in the reinjury rate. We conclude that functional treatment is safe, associated with a more rapid recovery, and particularly suitable in athletic populations.