Assessment of clinical instability of thoracolumbar burst fractures remains controversial and subjective. The purpose of the study was to obtain objective measures of acute instability of these fractures. Thirteen fresh cadaveric human spine specimens (T11-L1) were subjected to high-speed axial trauma, resulting in burst fractures in 10 specimens. Multidirectional flexibilities were measured when the specimen was intact and after the trauma. The average ranges of motion of the burst fractures, measured as percentages of the corresponding intact values at 7.5 Newton-meters, were 202%, 403%, 266%, and 462% for flexion/extension, axial rotation, lateral bending, and tension/compression, respectively. For the neutral zone motion parameter, the motions of the burst fracture were even greater: 670%, 1650%, 779%, and 650%, respectively. All of the increases were significant (P < 0.05). The clinical significance of the study lies in its finding of high multidirectional acute instability of the thoracolumbar burst fracture, especially in axial rotation.