STUDY DESIGN: This study analyzed the efficacy of the reduction and its maintenance using the posterior pedicular screw system in adult patients undergoing surgical treatment for symptomatic, isthmic spondylolisthesis. OBJECTIVE: To evaluate the effect of the reduction for lumbar isthmic spondylolisthesis in adult. SUMMARY OF LITERATURE REVIEW: If the increase of the shear force is approved as a main biomechanical feature of spondylolisthesis, reduction of the shear forces to the physiologic level must be regarded as the goal of surgical treatment. MATERIALS AND METHODS: 11 cases were treated with posterior monosegmental two point f;cation and posterolateral fusion (Group 1), 8 cases with a combined surgery of posterior monosegmental two point fixation, posterolateral fusion and anterior interbody fusion (Group 2), and 20 cases with posterior two segmental three point fixation and posterolateral fusion (Group 3). We assessed the clinical results and the radiologic objective parameters. RESULTS: The patients were followed up for more than 2 years (mean : 64 months). The satisfactory clinical results were 9 cases (81.8%) in Group 1 ,7 cases (87.5%) in Group 2 and 17 cases (85%) in Group 3. Radiologically, the efficacy of reduction and its maintenance were satisfactory results in Group 2, and the changes of the slip angle and the disc height showed that the loss of correction was greater than postoperative correction, although there were no statistical significant differences in the three groups. CONCLUSION: According to the our results, there was no significant relationship between the clinical result and the rdiological one. However, we think that the correcton of the deformity using the posterior pedicular screw system have some biomechanical advantages and seem to be recommendable for the treatment of isthmic spondylolisthesis in adults.