Herniated disk and bulging annulus are the major causes of lower back pain. It is necessary to differentiate bulging annulus from herniated disk because of their different methods of treatment. Myelography is one of the useful diagnostic methods for disk diseases even though advanced diagnostic modalities such as CT and MRI are more accurate. Functional myelography is not a new technology except for two additional views flexion and extension are obtained with conventional myelography. Diffentiation between bulging annulus and herniated disk by conventional myelography is based on the extent and multiplicity of extradural deformity of the contrast filled dural sac and neural sleeve as well as the changes of nerve root. There is no previous report about differential points between bulging annulus and her-niated disk according to functional myelography. It is the purpose of this study to find any additional differential points on functional myelography between bulging annulus and herniated disk over conventional myelography. Authors analysed functional myelographic findings of 152 cases from July 1986 to July 1987. Among them 22 cases who had been suffered from cervical abnormality or vague lower back pain were diagnosed as normal by myelography and 30 cases of L4-5 herniated disk and 21 cases of L4-5 bulging annulus which had been finally diagnosed by operation were studied. The results were as follows. 1. In normal group anterior epidural space was gradually widened from the upper lumbar vertebra downward. And anterior epidural space was more widened at the disk level in extension view than in flexion except for L5-S1 level. 2. In bulging annulus group the shape of graph of anterior epidural space in flexion state was as similar as normal. Anterior epidural space in extension state was more widened at the buldging annulus than normal, but lesser than herniated disk. 3. In herniated disk group widening of anterior epidural space at the herniated disk level was persistent in both flexion and extension views and much more exaggerated in extension, 4. In herniated disk group anterior epidural space above the lesion was normal but widened at the vertebral body level just below the lesion.