Automated percutaneous lumbar discectomy (APLD) and chemonucleolysis represent two major modalities of recently popularized minimally invasive surgical procedures for the treatment of herniated lumbar discs. However, the controversies concerning the safety and efficacy of the procedures are yet to be settled. The purpose of retrospective study was to determine the efficacy of these percutanous procedures and hence to establish a rational guideline for the treatment of lumbar disc herniations. Clinical review of 615 patients treated by percutaneous procedures revealed 81% success rate following chemonucleolysis and 83% following APLD, with a mean follow-up of 38 months (range 24-60 months) and 30 months (range 24-38 months) respectively. APLD required less time for resolution of back pain, and resulted in less disc space narrowing than chemonulceolysis. Clinical failures of the procedures were associated with extruded or calcified discs, combined spinal stenosis and discitis. Reduction of herniation size in postoperative CT had no significant correlation to the clinical results. In conclusion, both procedures were effective in contained disc without calcification or associated spinal stenosis. This study suggests some advantages of APLD over chemonucleolysis in the aspect of rapidity of symptom relief, disc space narrowing and possible complications. Reduction in herniation size was not imperative for clinical success. Both procedures appear to be good alternatives to open discectomy in well selected patients, and may be good bridges for the wide gap between conservative treatment and open surgery.