Background: Degenerative spondylolisthesis is a common disease of the lumbar spine especially in older ones. The disease represents a challenge to the treating physician. At present, for those patients that deteriorate clinically, there are many proposed algorithms for the surgical treatment. This before and after study was undertaken to assess the surgical results of decompression and instrumented POSTEROLATERAL fusion in these patients.Methods: The study population consisted of 23 patients who had undergone no prior surgery for degenerative spondylolisthesis on the lumbar spine. These patients were treated by decompression, bilateral POSTEROLATERAL fusion, and segmental (pedicle screw) instrumentation with mean follow-up of 29 months (range, 13-73 months). Finally, the clinical results were evaluated for all patients by means of an Oswestry Disability Index (ODI) version 2.1, the Henderson’s functional capacity, and persistence of leg symptoms, low back pain or claudication. Mann-Whitney and Chi-Square tests were used to assess the average values and comparison, respectively.Results: Henderson’s functional capacity at the last visit session was excellent in 14 (60.9%), good in 7 (30.4%), fair in 2 (8.7%) cases. ODI decreased from 72.2% (50-88%) preoperatively to 14.4% (0-54%) at the latest follow-up visit. A history of leg pain or claudication was correlated significantly with the amount of decline in ODI score and Henderson’s functional capacity (p<0.05).Conclusion: In spite of limited number of our patients, decompressive surgery plus instrumented POSTEROLATERAL fusion is a safe, reliable, and satisfactory procedure for treating degenerative lumbar spondylolisthesis. This procedure may be done when conservative treatment was failed and psychological problems can be ruled out.