Various actions on the lumbar spine have been attributed to quadratus lumborum, but they have not been substantiated by quantitative data. The present study was undertaken to determine the magnitude of forces and moments that quadratus lumborum could exert on the lumbar spine. The fascicular anatomy of quadratus lumborum was studied in six embalmed cadavers. For each fascicle, the sites of attachment, orientation, and physiological cross-sectional area were determined. The fascicular anatomy varied considerably, between sides and between specimens, with respect to the number of fascicles, their prevalence, and their sizes. Approximately half of the fascicles act on the twelfth rib, and the rest act on the lumbar spine. The more consistently present fascicles were incorporated, as force-equivalents, into a model of quadratus lumborum in order to determine its possible actions. The magnitudes of the compression forces exerted by quadratus lumborum on the lumbar spine, the extensor moment, and the lateral bending moment, were each no greater than 10 per cent of those exerted by erector spinae and multifidus. These data indicate that quadratus lumborum has no more than a modest action on the lumbar spine, in quantitative terms. Its actual role in spinal biomechanics has still to be determined.