For diagnosis of instability in case of ruptures of the lateral ankle joint ligaments a special ultrasound examination technique was established. In 41 patients with acute ruptures and 8 patients with chronic instabilities a prospective preoperative sonographic instability-testing was performed and compared to intraoperative result. A sensitivity of 0.96 for injuries of the lig. fib. tal. ant. and 0.79 for the lig. fib. calc. proves the exactness of this method. This technique is easy to perform and the result can be documented. This examination is accepted by the patients.