Background Adjacent segment degeneration (ASD) following lumbar fusion has been well documented in recent years. However, the pathogenesis of ASD is not clear. To investigate this issue, we established a finite element model of segments L2-L5, simulated a single-segment posterior fixation in L3-L4 and investigated the stress variation and the effects of the instrumented lumbar posterior fixation on adjacent levels. Methods Models A, B and C of L2-L5 multisegment finite element intact models were established. In model A, segment L3-L4 was not fixed and was without disc degeneration in the adjacent segment (L2-L3, L4-L5); in model B there was posterior pedicle fixation in segment L3-L4 without disc degeneration in the adjacent segment (L2-L3, L4-L5); in model C there was posterior pedicle fixation in segment L3-L4 with a degenerated disc in the adjacent segment (L2-L3, L4-L5). Four levels of axial pressure, 0.3, 0.5, 1.0 and 2.0 MPa, were compared between each model of the stress variation on the discs of the adjacent segment (L2-L3, L4-L5). Results The maximum principal stress mean value of disc L2-L3 under four pressures in model A was determined. The statistical results showed that stress was not significant difference in disc L2-L3 between models A and B, but there was a significant difference in disc L2-L3 between models A and C and a significant difference between each group in disc L4-L5 under four pressure conditions. Conclusions The preoperative degeneration of the adjacent segment of the disc is a significant risk factor for ASD. Copyright (C) 2011 John Wiley & Sons, Ltd.