This study is concerned with biomechanical evaluations of isthmic spondylolisthesis that causes slippage of a defected vertebra. Because the frequency of this disease depends on the site of spondylolysis, it is supposed that the mechanical boundary condition in lower lumbar spine has influence on isthmic spondylolisthesis. To clarify the mechanism of isthmic spondylolisthesis, the effects of isthmic defect on stress distribution in intervertebral disks were investigated by using a three-dimensional finite element model of a whole spine. The results of extension motion analyses show that the maximum compressive principal stress of L 4-L 5 disk in L 4 defect model is significantly increased in comparison with that of an intact model and the instability of the defect vertebra is also increased. Such a high compression on the disk may cause disk degeneration which leads to spondylolisthesis.