Objective : To evaluate clinical and radiological results of two different fusion techniques in adult low-grade isthmic spondylolisthesis. Methods : Between November 2003 and December 2004, 46 consecutive patients underwent instrumented mini-transforaminal lumbar interbody fusion (mini-TLIF) (group I) at Wooridul Spine Hospital, Seoul, Korea. Between February 2003 and October 2006, 32 consecutive patients underwent instrumented circumferential fusion (group II) at Leon Wiltse Memorial Hospital, Suwon, Korea. The mean follow-up periods were 29.7 and 26.1 months, respectively. Results : Mean visual analog scale (VAS) scores for back and leg pain decreased, respectively, from 6.98 and 6.33 to 2.3 and 2.2 in group I and from 7.38 and 6.00 to 1.7 and 1.0 in group II. Mean Oswestry disability index (ODI) improved from 51.85% to 14.4% in group I and from 60% to 9.1% in group II. In both groups, VAS and ODI scores significantly changed from pre- to postoperatively (p<0.001), but postoperative outcome between groups was statistically not significant. Radiologic evidence of fusion was noted in 95.7% and 100% of the patients in group I and II, respectively. In both groups, changes in disc height, segmental lordosis, degree of listhesis, and whole lumbar lordosis between the pre- and postoperative periods were significant except whole lumbar lordosis in both groups. Conclusion : Clinical and functional outcomes demonstrate no significant differences between groups in treating back and leg pain of adult patients with low-grade isthmic spondylolisthesis. However, in terms of operative data (i.e. operation time and hospital stay), instrumented mini-TLIF demonstrated better results.