Since Hadra first used metalic internal fixation in the human spine with wire loop in 1895, various implants have been used. Knodt rod was designed on the principle that the distraction produced a localized flexion attitude in the area to be fused, thus increasing the size of the intervertebral foramina and decreasing the risk of intraforaminal encroachment on the nerve root. We have performed the posterolateral fusion with or without Knodt rod as a primary procedure in spondylolysis and spondylolisthesis of lumbar spine, which required arthrodesis of more than one level. For period of 8 1/2 years' from Sep. 1977 to Apr. 1986, 12 cases of diseased spine were treated with posterolateral fusion without Knodt rod, and 13 cases were treated with posterolateral fusion using Knodt rod. We compared with two groups, and the following results were obtained. 1. Solid bony fusion was 100% in posterolateral fusion with Knodt rod, and 92% without Knodt rod. 2. An average period of post-operative immobilization was 2 months in cases with Knodt rod, and 2.5 months without Knodt rod. 3. In posterolateral fusion with Knodt rod, 4 cases with the root symptoms were all improved. In posterolateral fusion without Knodt rod, 6 cases with root symptoms, were all improved. But in 1 case without the root symptoms, developed the root symptoms. 4. No significant changes in radiologic evaluation were found between posterolateral fusion with Knodt rod and without Knodt rod. 5. Post-operative complications developed in 4 cases (31%) with Knodt rod, and 6 cases (50%) without Knodt rod. 6. Clinically, the satisfactory results were 100% with Knodt rod, and 83% without Knodt rod.