Computerized axial tomography (CAT) in cases of stenosis of the lumbar bony vertebral canal may demonstrate features of diagnostic significance in the shape of the canal and the bony structures surrounding it. The distinguishing features are the measurements. Standardized measurements of the midsagittal diameters at the cephalad and caudad borders of the laminas of the vertebral canal are proposed as a means of furthering both surgical and comparative studies. Identification of these borders in CAT scans is described, as is the procedure for taking measurements. One or two interlaminar, transarticular CAT scans are also made. In these scans, the midsagittal diameters cannot be measured because the ligamentum flavum forms the posterior wall in the midline. The scans may, however, demonstrate narrow lateral recesses and deformities of the articular processes and joint spaces. Errors in measurement of midsagittal diameters result from the inability of CT body scanners to show the angle between axial sections and the anterior vertebral canal wall, and from the partial volume phenomenon. The diameters are then too large. Errors in the angle of axial sections also lead to distortion in reconstructions of sagittal sections of the vertebral canal from the soft material obtained from axial sections. New precision designs are described. The principal indication for the intrathecal injection of water-soluble contrast media in CAT scanning is the determination of the thickness of the ligamentum flavum. Apart from its value in distinguishing various forms of stenosis, CAT scanning may also reveal inadequacies in surgical decompression.