The organization of collagen in the supraspinous, interspinous, and longitudinal ligaments, as well as the ligamenta flava, in lumbar spines from human cadavers has been investigated by polarized light microscopy, scanning electron microscopy, and x-ray diffraction. These experiments were performed on ligaments in situ, with their bony attachments undisturbed, and on excised ligaments at a range of applied strains. Results were related to the composition (investigated by standard histologic techniques) and gross structures (investigated by light microscopy) of the ligaments. More importantly, the results were related to the mechanical properties of the ligaments, which include stiffness, failure conditions, stress relaxation, and hysteresis. Where necessary, results were supplemented by or compared with those obtained from pig ligaments. Mechanical properties were related to postural changes by ligament strains induced in cadaveric specimens, using results from the literature. Thus, ligament structures could be related to their physiologic functions.