Among the many causes producing lower back pain herniated nucleus pulposus and bulging disk have occupied large percentages and uniformly showed defect on lateral aspect of contrast filled thecal sac. But it is essential to differentiate each conditions from the other because of their different treatment methods. Differentiation at metrizamide myelography between a diffusely bulging disk (unlikely to casuse nerve root compression) and a herniated disk is bases on the curvature, extent, and multiplicity of theextradural deformity of the anterolateral margin of the contrast filled sac and onthe presence of fusiform widening of the most distal part of the affected nerve root. The deformities caused by a bulging disk are round, usually symmetrical (although occasionally more prominent on one side), do not extend above or below the disk space, and can show multiple level involvement ; the nerve root is uniform in caliber and normal in size (although some of severe bulging show fusi orm widening of the most distal part of the affected nerve root). The deformities caused by a herniated disk is angular and extends cephalad and/or caudal to the level of the disk space ; the affected nerve root is usually widened in its most distal visible part. A consecutive series of 50 patients with low back pain and no past history of back surgery who did metrizamide myelography under went spine CT and /or laminectomy. Using the criteria listed above was correct in 32(22 patients) of 34 (24 patients)(95%) surgically and/or computed tomographically confirmed bulging disks and in all 26(100%) surgically and/or computed tomographically confirmed herniated disks.