BACKGROUND: To evaluate the effect of spondylolisthesis on lumbar lordosis on the OSI (Jackson; Orthopaedic Systems Inc.) frame. Restoration of lumbar lordosis is important for maintaining sagittal balance. Physiologic lumbar lordosis has to be gained by intraoperative prone positioning with a hip extension and posterior instrumentation technique. There are some debates about changing lumbar lordosis on the OSI frame after an intraoperative prone position. We evaluated the effect of spondylolisthesis on lumbar lordosis after an intraoperative prone position. METHODS: Sixty-seven patients, who underwent spinal fusion at the Department of Orthopaedic Surgery of Gwangmyeong Sungae Hospital between May 2007 and February 2012, were included in this study. The study compared lumbar lordosis on preoperative upright, intraoperative prone and postoperative upright lateral X-rays between the simple stenosis (SS) group and spondylolisthesis group. The average age of patients was 67.86 years old. The average preoperative lordosis was 43.5degrees (+/- 14.9degrees), average intraoperative lordosis was 48.8degrees (+/- 13.2degrees), average postoperative lordosis was 46.5degrees (+/- 16.1degrees) and the average change on the frame was 5.3degrees (+/- 10.6degrees). RESULTS: Among all patients, 24 patients were diagnosed with simple spinal stenosis, 43 patients with spondylolisthesis (29 degenerative spondylolisthesis and 14 isthmic spondylolisthesis). Between the SS group and spondylolisthesis group, preoperative lordosis, intraoperative lordosis and postoperative lordosis were significantly larger in the spondylolisthesis group. The ratio of patients with increased lordosis on the OSI frame compared to preoperative lordosis was significantly higher in the spondylolisthesis group. The risk of increased lordosis on frame was significantly higher in the spondylolisthesis group (odds ratio, 3.325; 95% confidence interval, 1.101 to 10.039; p = 0.033). CONCLUSIONS: Intraoperative lumbar lordosis on the OSI frame with a prone position was larger in the SS patients than the spondylolisthesis patients, which also produced a larger postoperative lordosis angle after posterior spinal fusion surgery. An increase in lumbar lordosis on the OSI frame should be considered during posterior spinal fusion surgery, especially in spondylolisthesis patients.