The main symptoms of adult isthmic spondylolisthesis are frequently low back pain and radicular leg pain. Laminectomy and posterolateral fusion is somtimes unsuccessful because nerve roots are compressed by fibrocartilagenous tissue at pars defect, degenerated disc and slipped bony edge. The patients need complete neural decompression, reduction of slipping and stable fixation. We have operated 22 patients with PLIF using carbon cage and transpedicular screw fixation after posterior decompression by Gill's operation and complete removal of intervertebral disc. We studied the patients postoperatively to evaluate the symptomatic improvement, reduction rate of slipping and stability of lumbar spine. Twenty-two patients were operated from April, 1996 to June, 1997. Mean age was 46, ranging from 35 to 68. Mean follow-up duration was thirteen months. The levels of operarion were 9 at L4-5 and 9 at L5-S1, 4 at both levels. Mean preoprative slip was 17.2%, which was reduced to 11.3% postoperatively. Nineteen patients(86%) were satisfied with the result of operation. The fusion rate of PLIF was 86%. There was no breakage of instrumentation or postoperative instability. We consider that PLIF and transpedicular screw fixation for adult isthmic spondylolisthesis with radicular leg pain is a good methods to obtain complete neural decompression, reduction of slip and stable lumbar fixation.