In 20 consecutive patients submitted to CT evaluation of a suspected disk protrusion at level L4-5, CT measurements to define entry point and penetration angle for percutaneous diskectomy were performed. It was shown that in order to reach the disk center, the entry point had to be more lateral than recommended by most authors. Body height, weight and body surface area were not significantly correlated to entry point position or penetration angle. In particular, the extension of the intervertebral facet joint determined the route of the probe to the disk center. Therefore, CT-assisted planning of the instrument route prior to percutaneous lumbar diskectomy is helpful in defining the proper entry point and penetration angle for safe and effective instrument positioning.