Anteroposterior translation as a sign of segmental instability was documented by traction-compression radiography in the majority of lumbar segments presenting lytic or degenerative spondylolisthesis with normal disc space height. Severity of lower back pain symptoms correlated with the degree of instability, but not with the amount of static spondylolisthetic displacement. Unstable spondylolistheses exhibited decreased viscoelastic behavior with creep, the degree of the vertebral displacement being dependent on the amount and duration of load. Disc degeneration and traction spurs seemed to be factors that may stabilize spondylolisthesis, even at an early age.