Bone graft volume decreases postoperatively without known etiology. We sought to determine the bone graft volume over time in 15 consecutive patients undergoing a single-level, instrumented, posterolateral lumbar fusion for degeneration causing mechanical pain or spondylolisthesis, and to identify factors associated with bone graft resorption. Following Institutional Review Board approval, a retrospective analysis was performed. Immediate and 3-month postoperative lumbar spine CT scans were imported into imaging software for volumetric analysis. We found that the 15 patients averaged approximately 11% graft volume loss at 3 months postoperatively. All patients exhibited volumetric graft loss on each side (range, 0.3-45%). A paired t-test revealed that immediate postoperative graft volume on a patient's left or right did not reflect graft volume on that side 3 months postoperatively (p=0.0008). Gender, age, history of prior operation, history of regular exercise, body mass index, level fused, operative time, initial graft volume, and laterality did not influence percentage volumetric loss (p=0.1-0.5). Interestingly, people who smoked cigarettes (range, 10-40 pack-years) exhibited 27% graft loss, compared to 7% in those who did not (Spearman p=0.009 graft loss versus pack-years smoked). We concluded that bone graft exhibited resorption 3 months postoperatively on both sides of all patients in this series, and that smoking was significantly associated with increased bone graft resorption.