A significant number of patients complain of persistent or recurrent pain or neurologic symptom after low back surgery. Identification of the causes of the failure and salvage procedures for failed back surgery is a rising problem for orthopedic surgeons. The purpose of this study is to reveal the causes of failed back surgery, to report the result of the salvage procedure and to identify the possible methods to prevent failure. Forty-two patients who underwent salvage operation out of 687 patients of low back surgery from 1983 to 1990 were analyzed with an average follow-up of 1.7 year (range 1-4.3). Result: The causes of persistent or recurrent pain or neurologic symptom were as follows; persistent or aggravated lateral spinal stenosis due to segmental degeneration in 34 cases (81%), recurred herniation of intervertebral disc at the operated level in 21 cases (50%), herniation of intervertebral disc at the adjacent level in 12 cases (29%), instability such as pseudarthrosis or olithesis in 8 cases (19%) and complication associated with the instrument used in prior surgery in 1 case (2%). Complete posterior decompression and posterolateral fusion with pedicle screw fixation was undertaken in all cases and resulted in satisfactory outcome in 93%. Conelusion: The most common cause of failed back surgery was persistent or aggravated lateral spinal stenosis due to segmental degeneration. CT combined with MRI was mandatory for accurate preoperative evaluation of the spinal canal pathology. Posterior decompression including lateral recess and posterolateral fusion with transpedicular stabilization proved a definite role as a salvage procedure in the treatment of intractable low back disorders in degenerative spine. In cases of disc excision, thorough preoperative evaluation of any coexisting lateral stenosis and minimal operative damage to posterior element is essential for prevention of failure, and if posterior element is significantly damaged, fusion of the segment is mandatory. In surgery for stenosis, complete posterior decompression including lateral recess to prevent persistent lateral stenosis and solid fusion to prevent instability is essential to prevent failure.