Interbody fusion was performed on eight cases of lumbar instability;five spondylolisthesis and three spondylolysis. Five cases were operated by anterolateral approach through the retroperitoneal space, and posterior approach on three cases after Cloward. Symptomatic improvement was obtained in almost all cases and complete fusion was observed on the roentgenogram three months later. Ambulation was possible from the immediate postoperative day without external support. While posterior approach has an advantage to observe the nerve root under direct vision, anterolateral approach has the following advantages;It is less destructive to the spinal structures, It can be performed without teasing the neural elements, anterior slipping of the vertebral body can be reduced easily, and its was applicable to the upper lumber level where neural elements qre compact. We believe interbody fusion has definite advantage to obtain strong weight bearing stability than fusion of any other parts of the spine because it has rapid recovery, low morbidity and high percentage of complete cure without disability.