Purpose The primary objective of this systematic review is to compare the outcome after decompression with and without concomitant instrumented fusion in patients with lumbar stenosis and degenerative spondylolisthesis. Does adding fusion to simple decompression lead to better results? Methods PubMed, Embase, CENTRAL, Cochrane, Web of Science, CINAHL and Academic Search Premier were searched. All studies comparing outcome of decompression alone to decompression with concomitant-instrumented fusion in patients suffering from symptomatic lumbar stenosis with degenerative spondylolisthesis were included. Risk of bias was assessed using an adapted version of the Cowley checklist. Results Eleven studies were included in the analysis involving 3119 patients in total. In the majority of studies, including two RCTs, clinical outcome of both patient groups was comparable regarding most clinical outcome measures. Conclusion Currently there is not enough evidence that adding instrumented fusion to a decompression leads to superior outcomes compared to decompression only in patients with lumbar stenosis and degenerative spondylolisthesis. The most important clinical outcome measures, including the ODI, show comparable results. Therefore, the least invasive and least costly procedure, being decompression alone, is preferred in patients with low-grade spondylolisthesis with predominant leg pain. Take Home Messages 1. Adding fusion to a decompression does not lead to better outcomes regarding the ODI and leg pain. Graphical abstract2. Decompression with concomitant fusion is presumably associated with more complications compared to decompression alone.3. Decompression alone is preferred in patients with low-grade spondylolisthesis with predominant leg pain. Forsth, P., et al. (2013). Does fusion improve the outcome after decompressive surgery for lumbar spinal stenosis?: A two-year follow-up study involving 5390 patients. Bone Joint J 95-B(7): 960-965 Forsth, P., et al. (2016). A Randomized, Controlled Trial of Fusion Surgery for Lumbar Spinal Stenosis.