For the primary fusion of adult's lumbar spondylolisthesis the authors performed either posterolateral fusion(PLF) with transpedicular screw fixation, or posterior lumbar interbody fusion(PLIF) with cages and transpedicular screw fixation, on 75 patients during five years between Jan. 1993 and Dec. 1997. The outcomes were retrospectively reviewed with emphasis on comparison of the results of these two surgical methods. Between patients treated with PLF(PLF group) and PLIF(PLIF group) there were no differences in distribution of preoperative variables such as patients' age and gender, type, level, grade of spondylolisthesis, and symptom durations. Mean follow-up period was 22.3 months in PLF group(n=44) and 17.4 months in PLIF group(n=31) (p<0.001). The rate of fusion was 85% in PLF group, and 94% in PLIF group(p=0.292). The postoperative outcome, expressed as mean follow-up score of Prolo scale, was 6.86 in PLF group and 7.42 in PLIF group(p=0.007). The variables that were associated with the outcomes were the level of spondylolisthesis(p=0.001) and the type of fusion (p=0.023). Variables such as age, gender, type and grade of spondylolisthesis, preoperative symptom duration, radiological findings of fusion status, change in disc height, and change in sagittal angle were not associated with the outcomes. The complication rate was 11% in PLF group and 19% in PLIF group. Overall successful results, defined by 7 or more in follow-up score of Prolo scale, were seen in 68% of PLF group and 87% of PLIF group(p=0.098). It is concluded that better surgical outcomes were obtained by PLIF methods in adults with lumbar spondylolisthesis, but with regards to the incidence of complications, although mostly minor and reversible, was more frequent in PLIF group.