Spinal pseudarthrosis is an important mechanical complication in longstanding ankylosing spondylitis. Thirty-five patients with 40 lesions were studied. Three lesions through vertebral bodies resulted from complete fractures. The rest occurred at interspaces, more common at the lower thoracic and upper lumbar segments. Double pseudarthrotic lesions were observed in 5 patients. Progressive osteolysis of the anterior elements was prominent, with variable sclerosis, osteophytes, vacuum phenomenon, subluxation and fragmentation. A posterior element weak link, as a bony break or facet joint non-fusion, was an essential component in every lesion, playing an initiating or perpetuating role in its pathogenesis. Mechanical derangement from trauma, severe round kyphosis, spondylodiscitis, hip disease, spinal operation and unusual activities may be contributing factors. Initial treatment is conservative, but 16 patients required operative stabilisation.