Spinal pseudarthrosis in ankylosing spondylitis can be difficult to detect radiologically. This paper assesses the value of bone radioscintigraphy in relation to radiological examination. The patterns of tracer uptake in the spine were recorded in 16 patients in whom established ankylosing spondylitis was complicated by pseudarthrosis. Twenty such lesions were identified in 16 patients, mostly occurring at one level in a junctional region of the spine. Four patients had two lesions. Standard radiography failed to demonstrate this abnormality in seven patients and in these, tomography was required to confirm the scintigraphic findings. On the other hand, bone scanning was much less accurate in detecting the minor forms of marginal vertebral end plate destruction (spondylo-discitis) than conventional radiology. This study suggests that 99TcmMDP scanning is a useful primary investigation for the detection of spinal pseudarthrosis in patients with chronic ankylosing spondylitis who suffer late onset back pain.