Injuries to the lateral ligaments of the ankle require an accurate diagnosis, especially because most injuries to one ligament are not treated surgically any more. Different stress devices are in use for an objective and standardised assessment of the instability of the ankle joint. In a group of 76 patients with injuries to their lateral ligaments of the ankle we compared posttraumatic instability by radiological stress test as done by hand or by a Telos stress device. In 25 patients treated by surgery an additional intraoperative stress X-ray (talar tilt) was performed. The results of the instability tests done by hand versus those obtained with a Telos stress device showed a poor correlation (r = 0.786). An analogous result was obtained by comparing the Telos device stress views to the intraoperative stress controls done by hand (r = 0.771). Only the pre- and intraoperative measurements by hand showed a good correlation (r = 0.958). The results are discussed with reference to a biomechanical model and recommendations for routine diagnosis are proposed.