Transitional vertebrae include lumbarization and sacralization of lumbosacral region. The prevalence of transitional vertebra was reported as 3 to 21%. It is known that transitional vertebra is related to herniated nucleus pulposus and spinal stenosis but there is no report in English literature about the relationship of transitional vertebra to spondylolysis or spondylolytic spondylolisthesis. The purpose of this study is to evaluate the relationship of transitional vertebra to spondylolysis or spondylolytic spondylolisthesis and to find out the clinical relevance for the treatment. The cases included 182 cases of spondylolysis or spondylolytic spondylolisthesis who were treated at Severance hospital from 1987 to 1993. There were 33 cases of transitional vetebra; 12 lumbarization and 21 sacralization. And there were remaining 149 cases of control group. The degree of anterior sippage was measured by Meyerding's grading and percentage of Taillard method. As the results, the degree of average anterior slippage of L4 was 14.5% in the cases of isthmic defect in L4 and sacralization. The average slippage of L4 was 11.4% in the control group. The degree of average anterior slippage of L5 was 12.5% in the cases of isthmic defect in L5 and lumbarization, and 9.5% in the cases of isthmic defect in L5 and sacralization. The average slippage of L5 was 16.2% in the control group. In summary and conclusion, the cases with isthmic defect in L4 and sacralization showed more anterior slippage than the cases with isthmic defect in L4 without transitional vertebrae, and the cases with isthmic defect in L5 and sacralization showed less anterior slippage than the cases with isthmic defect in L5 without transitional vertebrae. It is concluded that more aggressive treatment is recommended in the cases of isthmic defect in L4 and sacralization, whereas more conservative treatment is recommended in the cases of isthmic defect in L5 and sacralization.