Posterolateral fusion has long been considered the "gold standard" technique for surgical treatment of adult spondylolisthesis. Superior results have subsequently been reported with interbody fusion with cages and posterior instrumentation. The goal of this prospective study was to compare the two techniques regarding their clinical outcomes and fusion rates. Fifty-two patients with isthmic spondylolisthesis were operated by the same surgeon. One group (25 patients) had decompression and posterolateral fusion (PLF) with a pedicle screw system ; patients in the other group were treated by decompression, posterior interbody fusion (PLIF) and a pedicle screw system. The two groups were similar with respect to grade of slipping, age, and activity. Seventy-seven percent of the patients had a good or very good result with PLIF and 68% with posterolateral fusion. However, there was no statistical difference in cases with low grade slipping, whereas the difference was significant for cases with high grade slipping. The fusion rate was 93% with PLIF and 68% with PLF, but without any significant incidence on the functional outcome. Based on these findings, we now use posterior interbody fusion for high grade spondylolisthesis which requires reduction or if the disc space is still high. When the slip grade is low, or the disc space is narrow, we prefer posterolateral fusion.