Seventy-eight patients were categorized as having degenerative instability in the lumbar spine, based on clinical symptoms and radiologic signs. Biplanar radiography was used to measure the angular and translational intersegmental motion components of flexion and extension of the lumbar spine. A comparison was made between this measured motion, the clinical symptoms, response to facet joint injection of anesthetic, and radiologic appearance of disc space and facets. The magnitude of the flexion motion and the magnitude of the anteroposterior (AP) shear motion accompanying the flexion was slightly less at symptomatic compared with nonsymptomatic levels. In most patients the AP shear motion at all levels was less than 3 mm (maximum 7 mm). The amount of forward shear motion correlated positively with the amount of flexion motion (r = 0.3). The shear-flexion ratio was significantly reduced at symptomatic levels of patients. Although this group of patients, taken as a whole, showed a tendency toward abnormal intersegmental motion of the lumbar spine, it was found that flexion-extension biplanar radiography was not useful in the diagnosis of lumbar instability.