Isthmic spondylolisthesis is a common cause of back pain and sciatica. Sometimes it is severe enough to choose surgical treatment. The purpose of this paper is to analyse the result of surgical treatment of isthmic spondylolisthesis concerning the fusion methods. From 1988 to 1994, twenty-six patients with isthmic spondylolisthesis were operated on by posterior decompression, pedicle screw fixation and fusion either by posterolaterally or PLIF. Twenty patients were followed for more than twelve months. Twelve patients were treated by posterolateral fusion and eight by PLIF only. There were two men and eighteen women. Age ranged from 28 to 59 (avg. 46.2). Time to follow-up was 12-63 months (avg. 27 months). L4 was involved in 11 cases and L5 in 8 cases. One patient had lesions on L3 and L4. Eleven patients(65%) had neurologic claudication, ten patients(50%) had motor deficit, fourteen(70%) had sensory deficit and six(30%) had loss of ankle jerk. All the neurologic symptoms were improved but ankle jerk was not improved in four patients. Clinical results, assessed by Kim's criteria, were excellent in four patients, good in twelve, fair and poor in two respectively. Three of the four excellent result were noted after PLIF and all of the four unsatisfactory result were noted after posterolateral fusion. Complications were donor site pain for bone graft in five, loss of reduction in two, screw failure in one and dural tear in one. In conclusion, the result of PLIF is superior to the posterolateral fusion for the treatment of isthmic spondylolisthesis.