Animal models are essential in spine research for evaluating implants and for studying spinal mechanics. Several studies have compared the geometrical characteristics of animal and human vertebrae, but few studies have compared the structure of the spinal ligaments. The purpose of this study was to systematically quantify the collagen fibre orientation of the porcine and human interspinous ligament and thereby allow clearer interpretation of function. Human and porcine lumbar spine segments were loaded with a 10 Nm pure-flexion moment and chemically fixed. The sagittal plane collagen fibre orientation in the mid-lumbar interspinous ligaments was quantified by examining histological sections using a plane-polarized light macroscope and custom analysis software. The specimens showed collagen fibres in a posterior-cranial orientation originating from the superior aspect of the spinous process of the inferior vertebra and merging into the supraspinous ligament. There were not any statistically significant differences in interspinous ligament collagen fibre orientation between the human and porcine specimens. The middle and ventral spaces between the spinous processes of the human specimens contained loose disorganized collagen, skeletal muscle, and voids. The main load-bearing component of porcine and human interspinous ligament at the midlumbar level appears to be the dorsal portion, which is oriented at approximately 77-79 degrees with respect to the mid-disc plane. This dorsal aspect has a long moment arm and therefore is well suited to prevent excessive flexion. The similarity of the interspinous ligament morphology suggests that the porcine lumbar spine is a good model of the human lumbar spine.