Object: The purpose of this study was to compare the methods of posterior lumbar interbody fusion (PLIF) and posterolateral fusion (PLF) in cases of isthmic Grades 1 and 2 lumbar spondylolisthesis, and to evaluate the clinical efficacy of the procedures.Methods: Operations were performed in 50 patients with lumbar spondylolisthesis in the authors' clinics between 2001 and 2007. Indications for surgery were low-back pain with or without sciatica and neurogenic claudication that had not improved after at least 6 months of conservative treatment.