Objective : To evaluate the clinical and radiological outcomes of lumbar interbody fusion and its correlation with various factors (e.g., age, comorbidities, fusion level, bone quality) in patients over and under 65 years of age who underwent lumbar fusion surgery for degenerative lumbar disease. Methods : One-hundred-thirty-three patients with lumbar degenerative disease underwent lumbar fusion surgery between June 2006 and June 2007 and were followed for more than one year. Forty-eight (361%) were older than 65 years of age (group A) and 85 (63.9%) were under 65 years of age (group B). Diagnosis, comorbidities, length of hospital stay, and perioperative complications were recorded. The analysis of clinical outcomes was based on the visual analogue scale (VAS). Radiological results were evaluated using plain radiographs. Clinical outcomes, radiological outcomes, length of hospital stay, and complication rates were analyzed in relation to lumbar fusion level, the number of comorbidities, bone mineral density (BMD), and age. Results : The mean age of the patients was 61.2 years (range, 33-86 years) and the mean BMD was -2.2 (range, -4.8 to -2.8). The mean length of hospital stay was 15.0 days (range, 5-60 days) and the mean follow-up was 23.0 months (range, 18-30 months). Eighty-five (64.0%) patients had more than one preoperative comorbidities. Perioperative complications occurred in 27 of 133 patients (20.3%). The incidence of overall complication was 22.9% in group A, and 18.8% in group B but there was no statistical difference between the two groups. The mean VAS scores for the back and leg were significantly decreased in both groups (p < 0.05), and bony fusion was achieved in 125 of 133 patients (94.0%). There was no significant difference in bony union rates between groups A and B (91.7% in group A vs. 95.3% in group B, p = 0.398). In group A. perioperative complications were more common with the increase in fusion level (p = 0.027). Perioperative complications in both groups A (p = 0.035) and B (p = 0.044) increased with an increasing number of comorbidities. Conclusion : Elderly patients with comorbidities are at a high risk for complications and adverse outcomes after lumbar spine surgery. In our study, clinical outcomes, fusion rates, and perioperative complication rates in older patients were comparable with those in younger populations. The number of comorbidities and the extent of fusion level were significant factors in predicting the occurrence of postoperative complications. However, proper perioperative general supportive care with a thorough fusion strategy during the operation could improve the overall postoperative outcomes in lumbar fusion surgery for elderly patients.