Lateral curvature is thought to cause asymmetrical loading of vertebral physes, resulting in asymmetrical vertebral growth according to the Hueter-Volkmann principle. A biomechanical model was used to calculate trunk muscle forces and intervertebral forces in a lumbar scoliosis and in a symmetrical spine, in order to quantify the degree of asymmetrical loading. The analysis included five lumbar vertebrae, the thorax and the sacrum/pelvis and 90 pairs of muscles. Five spinal geometries were analyzed: the mean spinal shape of 15 patients with left lumbar scoliosis having a Cobb angle of 38° and apex at L1-2 (reference or ‘100%’ geometry), and this geometry scaled such that it had 0%, 33%, 67%, and 132% of the lateral and axial asymmetry of the reference shape. The muscle and intervertebral forces for maximum efforts opposing moments applied to the T12 vertebra in each of the three principal directions were calculated. The loading state of each interspace was expressed as the resultant force, the lateral offset of the resultant force from the disc center, and the angle of the resultant force from the axial direction. These analyses indicate that lumbar scoliosis produces asymmetrical spinal loading characterized by shear forces tending to increase the scoliosis, but with little increase in the lateral offset of the resultant forces transmitted through motion segments. If scoliosis progression results from asymmetrical loading, it appears that it is the shear force component more than the lateral bending moment which is responsible for this.