Unstable pelvic ring fractures cause high rates of morbidity and mortality. Percutaneous iliosacral screws provide a safe pelvic fixation obviating large surgical exposures. The presence of transitional vertebrae may present difficulties in numbering and fluoroscopic identification of lumbar discs and vertebrae. We performed percutaneous iliolumbar screw fixation for an unstable pelvic fracture due to a traffic accident in a 20-year-old male patient with transitional lumbar vertebrae. No neurologic or vascular complications were seen and the patient returned to his preinjury work and was pain-free in the second postoperative year. In order to determine the anatomic relationship of the iliolumbar screw with the major neurovascular structures, a cadaveric study was performed on the corpse of an adult man. Following dissection, it was observed that the iliolumbar screw inserted from the lateral wall of the iliac wing passed through the iliopsoas muscle and advanced to the L, vertebral corpus posterior to the fourth and fifth lumbar roots and with a reasonably distant course from the common iliac veins and arteries.