A stress radiograph of the subtalar joint was taken using Telos equipment, while the X-ray beam was directed onto the posterior subtalar joint at angles of 30 degrees latero-medially and 40 degrees caudocranially. From this radiograph the subtalar tilt angle was measured as an index of stability of the subtalar joint. Intra-and interobserver evaluations of measurement of this angle were performed on 20 unstable ankles. The errors at a 95 per cent confidence level were 1.9 degrees for intraobserver A, 1.4 degrees for intraobserver B and 2 degrees in interobservation. Stability of the subtalar joint was evaluated on 46 lateral ligament injuries of the ankle (23 acute injuries and 23 chronic injuries) and 80 normal ankles. The subtalar tilt angle was 9.7 degrees +/ 3.2 degrees in acute injuries 10.3 degrees #/ 2.9 degrees in chronic injuries, and 5/2 degrees +/ 2/6 degrees in normal ankles. There were significant differences between the acute or chronic injury and the normal ankles. These results suggested that stability of the subtalar joint was disturbed following acute and chronic lateral ligament injuries of the ankle.