A 41-year-old Chinese woman with quiescent ankylosing spondylitis presented with increasingly severe, recurrent back pain. The thoracolumbar junction was focally tender on palpation. Radiographs and computerised tomography demonstrated T12/L1 pseudoarthrosis. Excision of pseudoarthrosis, followed by anterior and posterior spinal fusion were performed with good results. The clinical features, pathogenesis, and management of this complication are discussed. Imaging of pseudoarthrosis in ankylosing spondylitis is emphasized.