Objective To evaluate the mid-term cl inical outcome of instrumented sl ip reduction combined with 360° circumferencial fusion and restoration laminae for symptomatic adult isthmic spondylol isthesis. Methods Between October 2004 and March 2008, 44 patients with symptomatic isthmic spondylol isthesis underwent instrumented sl ip reduction combined with 360° circumferencial fusion and restoration laminae. There were 15 males and 29 females with an average age of38.4 years (range, 28-45 years). The disease duration was 14 months to 7 years (38 months on average). The affected vertebrae was L4-5 in 18 patients and L5, S1 in 26 patients. According to Meyerding’s grade for spondylol isthesis, 28 cases were rated as grade II and 16 as grade III. The visual analogae scale (VAS), Oswestry disabil ity index (ODI), and the short form 36 health survey (SF-36) scores were evaluated before operation and at last follow-up; the radiographical outcome was evaluated by measuring sl i pping percentage, heights of intervertebral space and foramen, and fusion rate. Results All patients were followed up 20-60 months (42 months on average). The VAS, ODI, and SF-36 scores were all significantly improved at last follow-up when compared with those before operation (P lt; 0.05). According to Morelos criteria, the cl inical results were excellent in 32 patients, good in 9, and fair in 3; the excellent and good rate was 93.2%. The preoperative average percentage of sl ip was 47.5%, which was improved to 2.6% 3 days after operation; the total average reduction rate was 97.4%, and it was maintained at last followup. The heights of intervertebral space and foramen were all improved significantly after operation (P lt; 0.05), and there was no significant difference between at 3 days after operation and at last follow-up (P gt; 0.05). X-ray and CT showed bony fusion 1 year after operation in all patients with a fusion rate of 100%. Compl ications included pain at donor site of il iac bone in 4 cases, superficial infection in 2 cases, dural tear in 1 case, and degeneration of adjacent vertebrae in 2 cases; no nerve root injury, pseudoarthrosis, failure of internal fixation, and acquired spinal canal stenosis occurred. Conclusion Instrumented sl ip reduction combined with 360° circumferencial fusion and restoration laminae is a rel iable procedure for adult isthmic spondylol isthesis with satisfactory mid-term results, a high fusion rate and low compl ication rate. The long-term outcomesshould be verified by follow-up in the future.