Numerous authors have stressed the importance of posterior arch, vertebral bodies, and intervertebral disk dysplastic changes in lumbosacral spondylolisthesis. An extensive morphologic analysis was conducted of the lumbosacral junction in a series of 100 consecutive patients presenting with severe lumbosacral spondylolisthesis. Statistical correlations were found between the pelvic incidence, sacral slope, and pelvic tilt, as found in healthy volunteers. A statistical relationship exists between the lumbosacral kyphosis and pelvic parameters that explains the global sagittal "balance" of this pathological posture. A well-defined morphological criterion, the "S1 index," strongly correlated with the severity and stiffness of lumbosacral spondylolisthesis.