STUDY DESIGN: This is a retrospective study. OBJECTIVES: We analyzed the risk factors and the surgical results for adjacent segment disease after lumbar fusion. SUMMARY OF LITERATURE REVIEW: Many studies have been performed about the risk factors for adjacent segment disease, but the findings are still controversial. MATERIALS AND METHODS: This study was carried out on 35 (13 men, 22 women) of 50 patients who underwent lumbar fusion due to adjacent segment disease with a minimum of 2 year follow-up period from July 1999 to July 2006. The differences of the interval to revision (IR) were statistically analyzed by the examining preexisting degenerative change in the adjacent segments on MRI, the number of fused segments, the lumbar lordosis and the sagittal balance. The surgical outcomes of reoperation were assessed by Brodsky's criteria. RESULTS: Junctional stenosis as adjacent segment disease was seen in 21 cases (60%) and instability was seen in 14 cases (40%), including 2 iatrogenic flat backs and 2 cases of lumbar degenerative kyphosis. The average IR was 93 months for the cases that had less than 2 segment fusion (20 cases) and 62 months in those with more than 3 segment fusion (15 cases). As for lumbar lordosis, 25 cases (71%) had a normal range of angle as well as 101 months until the IR and 10 cases (29%) had an abnormal range of angle as well as 64 months until IR. Six cases were beyond the normal range of sagittal balance (17%) and their average IR value was 59 months. Otherwise, the cases with a normal range of sagittal balance had 109 months for the IR. The clinical outcome was excellent in 6 cases (17%) and good in 15 cases (43%). CONCLUSION: To decrease the adjacent segment disease, we should seriously consider the extent of lumbar fusion and we should restore the angle in lumbar lordosis to the physiological range and the sagittal balance during the initial operation.