Be over 18 years of age.
Understand Norwegian language, spoken and written.
Have a spondylolisthesis, with a slip > = 3 mm, verified on standing plain x-rays in lateral view.
Have a spinal stenosis in the level of spondylolisthesis, shown on MRI, CT scan or myelogram.
Have clinical symptoms of spinal stenosis as neurogenic claudication or radiating pain into the lower limbs, not responding to at least 3 months of qualified conservative treatment.
Be able to give informed consent and to respond to the questionnaires.
Are not willing to give written consent.
Are participating in another clinical trial that may interfere with this trial.
Are ASA- grade > 3.
Are older than 80 years.
Are not able to fully comply with the protocol, including treatment, follow-up or study procedures (psychosocially, mentally and physically).
Have cauda equina syndrome (bowel or bladder dysfunction) or fixed complete motor deficit.
Have a slip > = 3 mm in more than one level.
Have an isthmic defect in pars interarticularis.
Have a fracture or former fusion of the thoracolumbal region.
Have had previous surgery in the level of spondylolisthesis.
Have a lumbosacral scoliosis of more than 20 degrees verified on AP-view.
Have distinct symptoms in one or both legs due to other diseases, e.g. polynevropathy, vascular claudication or osteoarthtritis.
Have radicular pain due to a MRI-verified foraminal stenosis in the slipped level, with deformation of the nerve root because of a bony narrowing in the vertical direction.