Magnetic Resonance Imaging (MRI) has been applied to the examination of changes in the length of the spine which occurs between the recumbent and upright positions. Disc volume changes and their correlation with the variation in standing height were examined in seven normal female volunteers with the results indicating that dynamic time-dependent deformations are present within the intervertebral disks. These are attributed to fluid flowing to and from the disk and by visco-elastic deformation of the anulus fibres under the influence of osmotic and mechanical pressures. A significant difference between the sexes in the maximum volume of the disks was present, attributed to the increased lumbar lordosis in the female. Spinal length changes between recumbent and upright postures and the associated sexual dimorphism are biomechanical and physical effects which may have major clinical significance in the context of scoliosis aetiology.