Segmental instability as a consequence of disc degeneration is presented as an entity capable of producing low-back pain which responds to surgical stabilization with spinal fusion, and is illustrated with a case report. Data from lumbar radiographs were analyzed with a computer, and the position changes of the lumbar vertebrae were computed relative to the neutral positions of each. The results indicate that segmental instability is a complex disruption of intervertebral motion, consistent with the coupling behavior of the spine.