Blplanar radiography was used to Investigate the threedimensional intervertebral movements of patients with symptomatic spondylolisthesis to assess whether there was evidence of instability. These movements were also compared with those of a group of normal subjects. The primary movements of flexion and extension were studied, and the three-dimensional analysis enabled intervertebral translations and any coupled movements of lateral bending and axial rotation to be examined. The results showed that symptomatic spondylolisthesis resulted in reduced mobility of all levels of the lumbar spine and the appearance of coupled movements at the higher levels. The pathology did not produce instability at the level of the slip in any of these patients.