The X-ray films of 40 patients with thoracolumbar fractures were studied in order to classify the fractures, to investigate the causes of neural compression, and to define which fractures were unstable. Each fracture could be classified as a wedge fracture, a burst fracture, or a fracture-dislocation. The fragment of bone that produced neural compression in wedge and burst fractures almost always arose from the upper half of the vertebral body and had a characteristic triangular shape, as viewed on lateral x-ray films of the spine. Based on radiographic evidence, fracture-dislocations, severe burst fractures, and wedge fractures associated with marked subluxation, traumatic spondylolisthesis, or horizontal fractures of a pedicle were considered to be unstable. Mild burst fractures and most wedge fractures were considered to be stable.