The mechanical properties of the human spine are best described by load-displacement curves which include coupling effects. Three-dimensional load-displacement curves were obtained for all levels of the human thoracic spine using fresh cadaver spines in an atmosphere containing 100 per cent humidity at 22 degrees centigrade to stimulate a physiological environment. Six forces and six moments were applied, one at a time, to the center of the upper vertebra while its subadjacent fellow was fixed. Assuming sagittal plane symmetry, vertebral displacement was measured in three-dimensional space and load-displacement curves were plotted for the main as well as the coupled motions. The thirty-six curves necessary to define the mechanical characteristics of each motion segment completely were determined for all eleven thoracic levels. The curves showed that all the thoracic spine is a complex three-dimensional structure with coupled motion characteristics. Axial forces (compression/tension) resulted in significant horizontal displacements. Spine motion segments were more flexible in flexion than in extension. The spine was found to be least flexible during axial compression.