As the population ages, more elderly patients suffer from spinal stenosis requiring lumbar fusion. However, there are few and conflicting results regarding the clinical outcome of lumbar fusion. The purpose of this study is to evaluate the safety and efficiency of posterior lumbar interbody fusion (PLIF) in over 75-year-old patients and analyze the relative effectiveness of lumbar spinal fusion surgery compared with decompression surgery for spinal stenosis. This retrospective review evaluated 25 patients aged 75 to 93 who were diagnosed with spinal stenosis and underwent PLIF for 24 months. The control group included 25 patients who were matched for age, gender, level, race, and severity of stenosis, and who underwent decompressive laminectomy and flavectomy without fusion (DLF). The fusion rate in the PLIF group was 32.0%, 84.0%, and 96.0% at 6, 12, and 24 months, respectively. During the follow-up period, 4 (16%) and 2 (8%) patients underwent revision surgery in the DLF and PLIF groups, respectively. The back pain in the DLF group decreased from 5.6 to 2.1 at 6 months and then substantially increased to 3.4 at 24 months. The decrease in back pain score after treatment was greater in the PLIF group compared to the DLF group (P < 0.01) with a statistically significant difference in the trend in the two groups over time (P < 0.01). Even in elderly patients, lumbar surgery appears to be a safe and justifiable treatment for spinal stenosis. Lumbar fusion surgery rather than decompressive surgery was recommended for those patients who mainly complained of back pain.