Data of the postmortem studies of the iliolumbar ligament are controversial because of the number, complexity, and variability of the structures present in the lumbosacral region. The objective of this work was to study the anatomy of the iliolumbar ligament to resolve some clinical problems: (1) do anatomic bases exist that can explain the lumbar painful syndrome termed "iliolumbar syndrome?" (2) do iliolumbar ligament varieties exist that can influence lumbosacral joint stability? Magnetic resonance was used to analyze the anatomic structure of the iliolumbar ligament of live human beings. Thirty iliolumbar ligaments of 15 volunteers were analyzed with magnetic resonance. The images were acquired along the transversal and coronal planes (respectively, superoinferior and anteroposterior). The portion of the iliolumbar ligament originating from the L-5 transverse process is made up of two bands (anterior and posterior). The anterior band is broad and flat and has two different anatomic varieties. Type 1 originates from the anterior aspect of the inferolateral portion of the L-5 transverse process and fans out widely before inserting on the anterior portion of the iliac tuberosity. Type 2 originates anteriorly, laterally, and posteriorly from inferolateral aspect of the L-5 transverse process and fans out before inserting on the anterior portion of the iliac tuberosity. The posterior band of the iliolumbar ligament originates from the apex of the L-5 transverse process and is fusiform. Just before inserting on the anterior margin and apex of the iliac crest it widens, assuming the aspect of a small cone. On the transaxial plane, the anterior band of the iliolumbar ligament was placed along the horizontal line passing through the transverse processes, whereas the posterior band formed an angle of approximately 45 to 55 degrees opened posterolaterally with this line. On the coronal plane, the spatial disposition of the iliolumbar ligament varies greatly with the size of the L-5 vertebra and its position in the pelvis: (1) when L-5 is situated low in the pelvis, the bands of the iliolumbar ligament are longer and oblique; (2) when L-5 is situated high in the pelvis, the bands of the iliolumbar ligament are shorter and horizontal. The insertion manner of iliolumbar ligament posterior band in the iliac crest allows us to confirm the possibility of existence of the lumbar painful syndrome termed iliolumbar syndrome and confirms the possibility of examining its insertional site manually. Being accessible manually, various drugs can be injected directly into it or deep friction can be applied. This posterior band is thinner than the anterior, with a smaller insertional base on the iliac crest, which explains its lesser resistance to torsional overloading and also explains the frequency of this painful syndrome. It is probable that the spatial disposition of the iliolumbar ligament influences its antitorsional role. Further anatomic and biomechanic studies are needed.