Methods: We included 62 patients who underwent combined LLIF surgery and PPS fixation for degenerative lumbar spondylolisthesis with spinal canal stenosis. We compared the patient demographics and the accuracy of fluoroscopy-guided PPS placement between two groups: patients who remained in the lateral decubitus position for the pedicle screw fixation (single-position surgery [SPS] group) and those who were turned to the prone position (dual-position surgery [DPS] group). Results: There were 40 patients in the DPS group and 22 in the SPS group. Of the 292 PPSs, only 12 were misplaced. In other words, 280/292 screws (95.9%) were placed correctly in the pedicle’s cortical shell (grade 0). PPS insertion did not cause neurological, vascular, or visceral injuries in either group. The breach rates for the DPS and SPS groups were 4.1% (grade 1, 5 screws; grade 2, 3 screws; grade 3, 0 screw) and 4.1% (grade 1, 2 screws; grade 2, 2 screws; grade 3, 0 screw), respectively. Although there were no statistically significant differences, the downside PPS had more screw malpositioning than the upside PPS. Conclusions We found that PPS insertion with the patient in the decubitus position under fluoroscopic guidance might be as safe and reliable a technique as PPS insertion in the prone position, with a misplacement rate similar to that previously published.