The authors examined factors affecting fusion rate in the surgical treatment of 89 consecutive adult patients with spondylolisthesis. Two factors significantly improved fusion rate: combined anterior and posterior fusion and rigid postoperative immobilization in the cast. In 65 patients with isthmic spondylolisthesis, the fusion rate was raised from 70% when posterior fusion alone was used to 88% when combined anterior and posterior fusion was used. In 20 patients with degenerative spondylolisthesis, frequent use of combined anterior and posterior fusion contributed to a high overall fusion rate of 95%. Among patients with isthmic spondylolisthesis, postoperative cast immobilization resulted in a higher fusion rate of 90% compared with a fusion rate of 63% obtained after brace immobilization.