Background: Adjacent segment disease (ASD) is a major complication following spinal instrumentation and fusion. The search for of the rod flexibility factors responsible for junctional degeneration is still ongoing. Objective: Determine the rod stiffness and ASD following posterior instrumentation and fusion for lumbar spine and find the proper rod diameter for adult spinal instrumentation for fusion. Subject and methods: Retrospective evaluation of all patients requiring spinal instrumentation to determine the different rod diameter that predispose toward junctional degeneration was completed. All patients requiring spinal instrumentation over a one-year period were studied retrospectively. One-hundred eight-seven patients (mean age 61.6 years) who had undergone decompression and fusion with pedicle screw instrumentation were evaluated. The average follow-up was 4.2 years. The average number of levels fused was 2.9 segments (range: 1-8). Adjacent spinal level pre- and post-operatively was determined on the plain X-rays. Junctional degeneration was defined as new episode of degeneration of the adjacent level on radiologic finding. Asymptomatic patients did not demonstrate junctional degeneration on the routine post-operative X-rays. Results: ASD developed in 15 (8.0%) out of 187 patients, including compression fractures (n=2), spinal stenosis (n=6), and symptomatic disc collapse (n=7). There was a close correlation between the posterior instrument stiffness and the development of ASD (p=0.011). For fusion and fixation with 5.5 mm and 6.0 mm rod diameter, ASD occurred in four (3.7%) out of 108 patients and in 11 (13.9 %) out of 79 patients, resepectively. The incidences of ASD were greater when the posterior instrument used were stiffer in lumbar spine fusion. The pre-operative age, gender, and indication for surgery were not associated with the development of ASD. Conclusion: The prevalence of symptomatic ASD relatively increased with increasing stiffness of spinal implant. The diameter of the longitudinal rod strongly affected the fixator loads, and influenced the stresses in the vertebral endplates. The rod diameter had influence on the stresses in the adjacent spinal motion segment.