The Dunham spondylometer is described and a range of normal values for its use is established. A short retrospective study of patients with ankylosing spondylitis whose progress has been monitored using this instrument is reported. The range of extension in spondylitics was found to be consistently below the value obtained for the normal population. Total range of spinal mobility in the spondylitic group decreased slowly over a period of fourteen years. The need for prospective studies of spinal mobility in ankylosing spondylitis is emphasized.