Aims: To investigate the functional outcome with different autologous bone graft weights in posterolateral lumbar spine instrumentation for DSD’s. Methods: A total of 79 patients (mean age of 47.2) with chronic lower back pain for at least 2 years, with at least 1 year of failed conservative treatment and who were undergoing pos-terolateral spinal instrumentation for DSD’s were admitted to the trial. Patients were randomised into three groups to receive bone graft weights of 25g (Group A=26 patients), 50g (Group B=30 patients) and 100g (Group C=23 patients) per segment. Pain, clinical characteristics, disability (Oswestry), radiographic fusion, patient satisfaction and complications were recorded at 3, 6, 12 and 24-month follow-ups. Results: At 2 years, Group B (50g) had a significantly improved Oswestry Index, pain intensity, motor and sensory changes, and overall patient satisfaction than Group C (100g), which was significantly better than Group A (25g). There was no correlation between bone graft weight and radiographic fusion, between fusion and outcome, nor smoking and fusion or bone graft weight. Complications were similar. Conclusions: Patients had significantly better functional and clinical outcomes with bone graft weights of 50g. Bone graft weights of 25g had the worse outcomes and 100g did not seem to have any beneficial affect over 50g. We recommend autologous bone graft weights of 50g per fused spinal segments in this group of surgical patients.