In Iumbar region the epidural fat pad is relatively abundant so that CT can provide sufficient informations in diagnosis of lumbar HNP. Many authors have reported on the CT findings of HNP such as focal nodular protrusion of the posterior disc margin, obliteration of epidural fat pad, impingement of dural sac and nerve root, swelling of nerve root, soft tissue density in the spinal canal and calcification of disc. However three was no previous report describing indidence and reliability of these findings. It is the purpose of the present study to survey the frequency, reliability, and limitation of these CT findings. The clinical material was consisted of 30 operatively proven cases of HNP of the lumbar spine. Each lumbar CT scan was reviewed retrospectively and the findings were analysed by two radiologists independently. there were 20 males and 10 females and the mean age was 36.7 years. Involvement of L4-5 level was 2.3 times more frequent than that of L5-Sl level. Of 30 cases, 22 were unilateral posterolateral types and 8 cases central or unilateral far lateral types. CT findings observed were nodular protrusion of the posterior margin of the disc, obliteration of epidural fat pad, impingement of dural sac or nerve root, soft tissue density in the spinal canal and cal- cification in the posterior portion of the protruded disc, in order of decreasing frequency. The conclusions are as follows : 1. Nodular protrusion of the posterior disc margin accompanied by obliteration of epidural fat pad was observed in every case. The former finding was designated as direct sign and the latter indirect. 2. Obliteration of the epidural fat appears to be significant in lateral recesses especially when it occurs unilaterally. This was not true, however, in the centraly located fat pad. 3. Impingement of the dural sac and nerve root were observed in 90% and 67%, respectively, and were very helpful in establishing HNP diagnosis when the direct and indirect signs were equivocal. Metrizamide CT scan facilitated the evaluation of the dural sac and nerve root. 4. Soft tissue density in the spinal canal and calcification in the posterior portion of the protruded disc were demonstrated in only 2% and 1% of the cases, respectively.