100 patients with low-grade isthmic spondylolisthesis were randomized to undergo a single-level instrumented posterior lumbar interbody fusion with either one cage or two. The results after a two year follow-up indicated similar fusion rates, complications rates, and clinical outcomes, measured using the Oswestry Disability score and pain scores between both groups. Examination of operative characteristics and an immediate postoperative follow-up demonstrated that instrumentation with a single cage resulted in significantly shorter mean operative time, and less blood loss measured over the first post operative day.