Severe and persisting pain and disability due to a degenerative narrowing of the spinal canal, lumbar spinal stenosis, can be operated with a simple surgical decompression. Sometimes, there is also a slippage of vertebra, degenerative spondylolisthesis. In such cases, instrumental stabilization (e.g. screws and rods) has been recommended. Even though additional fusion is more complex and riskier, and evidence in high-quality Scandinavian studies shows that it is unnecessary, decompression plus fusion is still the treatment of choice in the USA and most European countries. This reluctance to change clinical practice is mainly due to concerns about long-term results, especially higher reoperation rates among patients operated with decompression only. This register-based non-inferiority study aims to assess long-term reoperations among those treated with and without additional fusion surgery.