Surgical decompression is the recommended treatment for patients with moderate to severe degenerative lumbar spinal stenosis (DLSS). Although complication risk has been shown to be higher with concomitant fusion, the success rate is not necessarily superior. This study analyzed the success rates of 58 DLSS patients treated with decompressive surgery. Twenty patients received concomitant instrumented fusion. Outcomes were measured with the Swiss Spinal Stenosis Questionnaire (SSSQ) completed pre-operatively and at least 12 months post-operatively (range 12 to 54 months). Overall, 63.8% of the patients had significant clinical improvement in Symptom Severity, 55.2% had significant clinical improvement in Physical Function, and 58.6% of the patients were at least somewhat satisfied; 43.1% (25/58) of the patients met all three criteria and were considered to be clinically successful. There were no statistically significant differences between the clinical success rates of the non-fusion and fusion groups, but the change in mean change of the Symptom Severity score for the fusion group was significantly greater than that of the non-fusion group. Also, patients with more severe pre-operative symptoms and more physical function restrictions had better success results than those patients with more mild symptoms and less restrictive physical function. The results of this study demonstrate that decompressive surgery with concomitant fusion does not impose a greater risk than decompressive surgery alone and the clinical results of the added fusion are somewhat superior to decompressive surgery alone.