A 41-year-old male with a 20-year history of classical ankylosing spondylitis, psoriasis and seropositive, nodular erosive rheumatoid arthritis presented with a 12-month history of thoracolumbar junction pain following minor trauma. A pseudoarthrosis was noted at the T11/12 level on plain radiographs and tomograms. A gallium scan showed no increased isotope uptake, and a computed tomogram (CT) revealed no evidence of a paraspinal collection. Conservative management including cast immobilisation and local radiotherapy was ineffective, and spinal fusion was required. A typical Andersson lesion was found at operation. The diagnostic and therapeutic problems of such discovertebral lesions are discussed.