Functional instability, i.e., recurrent sprains or a feeling of "giving way" of the ankle, is common after ankle sprains. These patients have been supposed to have peroneal muscle weakness based on manual methods for measuring muscle strength. We have confirmed this theory by an isokinetic method for testing pronator muscle strength. Pronator muscle weakness on the affected side was found among patients with unilateral functional instability of the ankle joint. Impaired postural control was demonstrated by stabilometry with the patient standing on either foot.