The majority of herniated intervertebral disc of lumbar spine do respond to conservative management, and only 20% requires surgical approach such as open discectomy, chemonucleolysis, or automated percutaneous lumbar discectomy. Open discectomy is useful for most patients but still carries the risks of general anesthetic, injury to soft tissues, joints, neural structures, and with postoperative epidural fibrosis or scarring. Because of this, automated percutaneous lumbar discectomy, by mechanically decompressing the disc, has been advocated as an alternative treatment of herniated intervertebral disc in selected patients. We reviewed 100 patients treated by open discectomy from Jan. 1986 to Dec. 1990, and 39 patients by percutaneous lumbar discectomy from Jan. 1990 to Apr. 1991, of which duration of follow-up in each group was 18.0 months and 14.9 months respectively. Changes in the objective and subjective signs and symptoms of patients were examined preoperatively and postoperatively. The results were as follows; 1. Most frequent level was L4-5 (64.0%, 51.3%): 2 level involvement was 17.0% and 17.9%: 3 level was 1 case in each group. 2. Average time of each procedure was 127.2 minutes and 24.9 minutes. 3. Average hospitalization period was 14.2 days and 3.5 days. 4. verall clinical results were similar for two groups. In open discectomy, 88 cases (88.0%) showed excellent or good results. In percutaneous discectomy, 32 cases (82.1%) showed same results. 5. In percutaneous lumbar discectomy, volume of excised disc was 3.8cc in successful cases, whereas 2.9cc in unsuccessful cases. 6. In percutaneous lumbar discectomy, 7 cases (16.3%) showed fair or poor results. Causes of them were combined stenosis, subannular herniation with large size, subligmanentous heniation without migraration and with migration, recurred disc, and predictably compensatory reaction. 7. In open discectomy, 12 cases (11.8%) showed unsatisfactory results. Causes of them were perineural adhesion or scarring, herniation at another level, recurred disc, and compeusatory reaction. 8. There was no significant complications postoperatively in each group. From the above observation, the most important factors in the success of this procedure are using strict criteria for patient selection.