PURPOSE: To clarity the features of vertebral pseudarthrosis and to evaluate the efficacy of posterior instrumentation and fusion in treatment of it. MATERIALS AND METHODS: Twelve patients with severe back pain and intravertebral pseudarthrosis showing vacuum phenomenon were treated by in situ posterior instrumentation and fusion. The kyphotic angle of pre- and post-treatment was measured on a lateral radiograph. MRI was performed in all patients. The intravertebral instability was confirmed from the dynamic lateral view. The pain level was assessed both before and after the treatment using a visual analog scale. The kyphotic angle at last follow up was also checked. RESULTS: In eight cases, intravertebral instability was shown at the clefts in flexion-extension radiographs. MRI showed that the cleft was low intensity on the T1- weighted image and high intensity on the T2-weighted image. The pre-and postoperative mean kyphotic angles were -18.3degrees and -8.5degrees respectively. The preoperative average pain score was nine and postoperative four. At last follow up the mean kyphotic angle was -16.6degrees CONCLUSION: The cleft with intravertebral vacuum phenomenon and magnetic resonance findings of low intensity on the T1-weighted scans and high intensity on the T2-weighted scans suggests that the cleft is a pseudarthrosis associated with avascular necrosis of the vertebral body. The posterior instrumentation and fusion provides satisfactory pain relief in patient with vertebral pseudarthsosis.