Twenty seven patients of symptomatically unstable spondylosis or spondylolisthesis, that failed respond to conservative treatment, were treated with a simplified unilateral posterior lumbar interbody fusion in which the disc was removed and replaced by two bone dowels and chip bones. The symptomatic spinal canal was decompressed by removing lamina and/or unroofing the intervertebral foramen, unilaterally or bilaterally, removing disc, and decorticate partially the adjacent vertebral bodies in semicirculr fashion unilaterally. The exposed intervertebral tunnel was packed with chip bones and two biocortical bone dowels serially, and reinforced by small bone screw. The clinical status and stability were verified by clinical examination and motion roentgenographic films during follow up period, which ranged from 8 to 20 months. Symptomatic improvement was observed in 23 cases (85%) and solid fusion was confirmed by stress view and computed tomography at 3 or 6 months after operation in 24 cases (89%). The complication rate was 41%(11 cases) in which all but one cases of nerve injury were resolved within 3 months while reoperation was needed in 3 cases.