Purpose <p>The aim was to investigate if spinal canal stenosis in the adjacent segment of a primary single-segment fusion surgery should be treated by decompression only or decompression plus fusion.</p> Methods <p>This study is a retrospective cohort study. Inclusion criteria were all adult patients with spinal canal stenosis without relevant foraminal stenosis in the adjacent segment when undergoing primary single-segment lumbar fusion surgery with a followup of at least 1 year. One group was treated with decompression only of the adjacent segment, while the other group underwent decompression plus fusion of the adjacent segment. The primary outcome was the Oswestry Disability Index (ODI) pre- and postoperatively. Secondary outcomes were visual analogue scale (VAS) back/leg and revision surgery.</p> Results <p>There were no differences in baseline data when comparing the decompression only group (<i>n</i> = 53) with the decompression plus fusion group (<i>n</i> = 68) regarding age, gender, body mass index, American Society of Anaesthesiologists classification, Frailty Index, and smoking. The change of ODI from pre- to postoperatively at 3 months was significantly better for the decompression only group (− 28 [IQR 20] versus − 22 [IQR 20], <i>p</i> = 0.047, linear regression coefficient 7 [95% confidence interval 1.2–12.3], <i>p</i> = 0.02). This difference is diluted to similar ODI values at final followup after 15 (IQR 15) months. Other clinical outcomes and revision rates were similar in both groups.</p> Conclusions <p>Patients with lumbar spinal canal stenosis without foraminal stenosis in the adjacent segment show no significant difference with adjacent segment decompression versus additional fusion when undergoing a primary single-level fusion. Decompression only of the adjacent segment could potentially be favored to opt for a slightly better short-term clinical outcome after 3 months.</p>