OBJECTIVE: We used a finite element (FE) analysis to investigate the biomechanical changes caused by transforaminal lumbar interbody fusion (TLIF) at the L4-L5 level by lumbar lordosis (LL) degree. METHODS: A lumbar FE model (L1-S5) was constructed based on computed tomography scans of a 30-year-old healthy male volunteer (pelvic incidence,= 50°; LL, 52°). We investigated the influence of LL on the biomechanical behavior of the lumbar spine after TLIF in L4-L5 fusion models with 57°, 52°, 47°, and 40° LL. The LL was defined as the angle between the superior end plate of L1 and the superior end plate of S1. A 150-N vertical axial preload was imposed on the superior surface of L3. A 10-N/m moment was simultaneously applied on the L3 superior surface along the radial direction to simulate the 4 basic physiologic motions of flexion, extension, lateral bending, and torsion in the numeric simulations. The range of motion (ROM) and intradiscal pressure (IDP) of L3-L4 were evaluated and compared in the simulated cases. RESULTS: In all motion patterns, the ROM and IDP were both increased after TLIF. In addition, the decrease in lordosis generally increased the ROM and IDP in all motion patterns. CONCLUSIONS: This FE analysis indicated that decreased spinal lordosis may evoke overstress of the adjacent segment and increase the risk of the pathologic development of adjacent segment degeneration; thus, adjacent segment degeneration should be considered when planning a spinal fusion procedure.