Transitional lumbar vertebrae include lumbarization and sacralization of the lumbosacral region. This study was performed to evaluate the relationship of transitional vertebra to spondylolytic spondylolisthesis such as the incidence and degree of slippage and to ascertain the clinical relevance for treatment. The study included 182 cases with 33 cases (18.1%) of transitional vertebra, 12 cases of lumbarization, and 21 cases of sacralization. The remaining 149 cases constituted the control group. The degree of the anterior slippage of the vertebral body was measured by Meyerding's grading and the percentage of the anterior slippage was measured by Taillard's method. In the patients with lumbarization and the isthmic defects in the fourth lumbar spine, the average slip of L4 was 14.5%. While patients with sacralization and the isthmic defects in L4, the average slip of L4 was 19.3%. The average slip of L4 was 11.4% in the control group. In patients with lumbarization and the isthmic defects in the fifth lumbar vertebra, the average slip of L5 was 12.5%. While in patients with sacralization and the isthmic defects in L5, the average slip of the L5 vertebra was 9.5%. The average slip of L5 was 16.2% in the control group. The patients with sacralization and the isthmic defects in L4 showed more anterior slippage than the patients with the isthmic defect in L4 without transitional vertebrae. The patients with sacralization and the isthmic defects in L5 showed less anterior slippage than the patients with isthmic defects in L5 without transitional vertebrae. From this it can be concluded that more aggressive treatment is recommended in the patients with sacralization and isthmic defects in L4, whereas more conservative treatment is recommended in the patients with sacralization and the isthmic defects in L5.