Deformities of the lateral margins of the contrast material-filled lumbar thecal sac are common findings at myelography in patients with low back pain, but not all such deformities are due to herniated disks. Differentiation at Amipaque myelography between a diffusely bulging disk (unlikely to cause nerve root compression) and a herniated disk (which typically causes nerve root compression) is based on the curvature and extent of the extradural deformity of the anterolateral margin of the contrast-filled sac and on the presence of fusiform widening of the most distal part of the affected nerve root. The deformity caused by a bulging disk is rounded, usually symmetrical (although occasionally more prominent on one side), and does not extend above or below the disk space; the nerve root is uniform in caliber and normal in size. The deformity 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 33 patients with clinically suspected lumbar disk herniation and no previous history of back surgery underwent laminectomy. Using the criteria listed above for differentiation of bulging from herniated disk on Amipaque myelography, the myelographic diagnosis was correct in all six operatively confirmed bulging disks and in 26 (96%) of 27 operatively verified disk herniations.