Injuries to the lateral ligaments of the ankle require accurate diagnosis, especially because most injuries to a single ligament are not treated surgically any more. Various stress devices are in use for objective and standardized assessment of the instability of the ankle joint. In a group of 76 patients with injuries to the lateral ligaments of the ankle, we compared post-traumatic instability by a radiological stress test as done by hand or by using 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 the Telos stress device correlated poorly (r = 0.786). A similar result was obtained by comparing the Telos stress device views to the intraoperative stress controls done by hand (r = 0.771). Only the pre- and intraoperative measurements by hand were in good correlation (r = 0.958). These results are discussed with reference to a biomechanical model and recommendations for routine diagnosis are proposed.