After Kilian first described apondylolisthesis in 1854, there has been diverse opinions over the surgical treatment of the spondylolisthesis: in situ fusion vs. reduction and fusion, PLIF vs. PLF, and instrumentation vs. non-instrumentation. The advantages of the instrumentation are rigid fixation, better union rate of grafted bone and early rehabilitation after surgery. The disadvantages are long operation time and increased chance of the wound infection. The purpose of this retrospective study is to review the clinical results and fusion rate of 41 patients who were treated with decompression and posterolateral fusion in situ with autograft with or without instrumentation from January 1986 to June 1992. 1. Among 41 patients, 20 patients were degenerative type and 21 were isthmic type, and most commonly involved level was L4-5(46.4%). 2. Fusion was obtained in all patients. 3. Clinically 11 of 15 patients(73.4%) showed excellent or good results in instrumentation group, and 19 of 26 patients(73.0%) showed excellent or good results in non-instrumentation group. 4. Postoperative complications developed in two cases(13.3%) with instrumentation and five cases(19.2%) without instrumentation. Comparing with the non-instrumentation, we could not find great advantage of the instrumentation on the clinical results or union rate, although instrumentation showed a little better result.