The iliolumbar ligament is one of the three lumbopelvic ligaments. Recent study has shown that the ligament is not present at birth and is formed from metaplasia of the quadratus lumborum muscle at the end of the first decade. To study the biomechanical functions of this ligament, an apparatus was developed using linear variable differential transformers. Twenty fresh cadaveric specimens of the lumbosacral pelvis were tested. The flexibility of the intact lumbar segment was studied before and after the ligaments were divided. Flexion of L5 on S1 was mainly controlled by the posterior band and lateral bending by the anterior band of the ligament. When the L5-S1 disc was degenerated, total division of the ligaments reduced extension during loading. Its effect on torsion was not evident from this study.