There are rare but serious complications-especially risk of paraplegia when instrumentation by surgery is used to correct a spinal deformity. Wake-up tests may be necessary during scoliosis or kyphosis surgery to ensure that spinal function remains intact. We tried four spinal fusions for ankylosing spondylitis of a 62 year-old woman. We were not able to fix the rod for fusion because of a presenting positive wake-up test in the previous two prone-positioned operations. In last operation we decided on normotensive anesthesia with fentanyl-propofol in a lateral decubitus position, and then surgical instrumentation was completed after we made sure of a negative weak-up test. There were no postoperative sequelae. The lateral approach to the thoracic disc space during spinal fusion may produce minimum disruption of the normal spinal musculoskeletal anatomy, avoid retraction of the spinal cord and preserve the neurovascular bundle and the segmental radicular arteries to the spinal cord.