STUDY DESIGN: A retrospective study. OBJECTIVES: We analyzed the risk factors and relevance of lumbar lordosis on the incidence of adjacent segment disease after posterior lumbar interbody fusion. SUMMARY OF LITERATURE REVIEW: Maintaining lumbar sagittal balance is important for decreasing the incidence of adjacent segment disease after posterior lumbar interbody fusion. MATERIALS AND METHODS: Among the patients who had undergone posterior lumbar interbody fusion of 1 or 2 levels between August 2001 and October 2008, we analyzed 153 patients who were available for at least three years of follow-up; among the subjects, 115 were males and 38 were females. Mean age among the patients at the time of initial surgery was 60.3 (range, 41-82) and mean followup period was 5.6 years (range, 3-11). The causative diseases were spinal stenosis in 78 cases, degenerative spondylolisthesis in 51 cases, isthmic spondylolisthesis in 23 cases, and degenerative disc disease in 1 case. At last follow-up, there were 52 cases (33.9%) of adjacent segment disease. Among them were found 21 cases (40.4%) of disc degeneration, 17 cases (32.7%) of instability, and 14 cases (26.9%) of simultaneous disc degeneration and instability. A total of 10 patients (6.5%) underwent a revision operation, and the mean period from initial to revision operation was 5.5 years (range, 3.1-10.3). We analyzed the correlation between risk factors of adjacent segment disease and the incidence of the disease depending on the gap between lumbar lordosis and pelvic incidence, and compared the clinical results of the 2 groups using modified Whitecloud classification. RESULTS: The incidence of adjacent segment disease was not statistically significant for patient age, sex, BMD, degree of obesity, causative disease, and the level of previous surgery. However, the incidence of adjacent segment disease was statistically higher in patients who had more than 9 degrees gap between lumbar lordosis and pelvic incidence (p=0.013). In our analysis of clinical results, 63.5% of the group of patients who developed adjacent segment disease and 80.2% of the group without adjacent segment disease had good or satisfactory results (p=0.031). CONCLUSIONS: Maintaining lumbar sagittal balance is important for decreasing the incidence of adjacent segment disease after posterior lumbar interbody fusion, and close observation is needed in patients with 9 or more degrees gap between lumbar lordosis and pelvic incidence.