The radicular canal is the lateral portion of the spinal canal when it is trefoil. It is a bony and ligamentary, monovertebral and indeformable space, the measurements of which are reproducible. The anatomy of this radicular canal has been studied in the whole of the lumbar vertebrae of 50 anatomical subjects. Its radiological anatomy has been defined by sagittal and computerised tomographies of these anatomical specimens, while sagittal tomographies were done for 25 control individuals devoid of symptoms. This canal, exceptional in the upper part of the lumbar canal, has been found in 72% of the cases in L4 and always in L5 and S1. Its sagittal diameter, which when measured must take into account the upper margin of the pedicle, has a theoretical minimal diameter of 3 mm to 3.8 mm, according to the vertebral level. 13% of the vertebrae were asymmetrical and no significant relationship exists between the median sagittal diameter of the spinal canal and the diameters of the radicular canals. Conventional sagittal tomography provides two types of images, according to the shape of the mouth of this canal and furnishes the best measurements (92% of the measurements were concordant), as long as the technique is followed closely. Transverse computerised tomography is less reliable, for the positioning of the section plane still remains the principal cause for error in measuring, but it provides a definition of the relations of the nerve elements with this canal. This radiological study will be suitable for sagittal reconstruction by computerised tomography, once this kind of examination can produce pictures of bones as detailed as those of conventional tomography.