Posterior lumbar interbody fusion(PLIF)for lumbar spondylolisthesis usually results in an excellent outcome, but the problem of so-called adjacent segment disease(ASD)remains unresolved. ASD at L3/4 was investigated in 85 patients with L4 spondylolisthesis treated by L4/5 PLIF with uniform pedicle screws and interbody cages who had been followed up for more than two years (mean, 38.8±17.1 months). The patients were divided into three groups according to the final outcome:a group without ASD(n=58), a group with radiological ASD(n=14), and a group with clinical ASD(n=13). Eleven patients in the third group were reoperated to treat L3/4 lesions. L4-5 disc height distraction by cage insertion in the three groups was 3.1 mm, 4.4 mm, and 6.2 mm, respectively, and the latter two values were significantly higher than the first value. Excessive distraction of the L4/5 disc space proved to be a significant risk factor for radiological and clinical ASD at L3/4.