An in vitro study was conducted to establish the thresholds of thoracic spine stability. Flexion or extension producing horizontal forces of 43 percent body weight were applied to fresh two-vertebrae spine specimens. Spine components were transected in two different sequences until failure. Load-displacement curves were measured. Intact spine exhibited average inter-vertebral horizontal translatory displacement of 1.0 mm (s.d. = 0.4) and sagittal plane rotation of 1.4 degrees (s.d. = 0.8). Just prior to failure these average displacements increased to 2.4 mm (s.d. = 1.4) and 4.1 degrees (s.d. = 1.7). A horizontal displacement of 2.5 mm (on lateral X-ray) or 5 degrees of angulation of one vertebra with respect to the other may indicate an unstable spine. This information, together with other clinical indicators of spine instability discussed here, will be helpful in the clinical judgment regarding an injured spine.