Segmental instability secondary to degenerative disc disease may result in chronic low-back pain. In the sagittal plane, segmental instability can be characterized during lumbar motion from full extension to full flexion. The authors studied this movement using a translational method for the kinematic analysis, implementing a new concept known as the instability factor. Both translational and angular components of motion are evaluated. By computing the incremental motion parameters at different stages of spinal bending, the total amount of translation and angulation is obtained and combined in a ratio termed the instability factor. This factor increases with linear instability and decreases with rotational instability. The authors reviewed 12 control subjects and 36 patients with chronic low-back pain. The diagnoses of patients were categorized into three groups: idiopathic low-back pain, lumbar disc prolapse, and degenerative disc disease. Lateral radiographs of each subject's spine at the L4-5 level were obtained using low dose radiography and were performed serially as the subjects moved from full extension to full flexion. It was found that the group of patients with degenerative disc disease had an average age-corrected instability factor of 37.3 (mm/radian), which was significantly larger than that of normal subjects 25.5 (mm/radian), (P = 0.0065). No significant difference was seen in the instability factor of patients with idiopathic low-back pain or lumbar disc prolapse.