The radiological appearances in twenty-five patients with Reiter's disease have been compared with those in eighty-one with rheumatoid arthritis and thirty-eight with ankylosing spondylitis. The similarities and differences have been analysed. Changes of periosteal new bone in the calcaneum appear to be the only significant differentiating factor, although minor differences in incidence and distribution occur elsewhere. Sacro-iliitis is a common radiological feature of long-standing Reiter's disease.