Sacroiliitis is an indispensable condition for the diagnosis of ankylosing spondylitis according to the present criteria and is usually diagnosed on standard anteroposterior radiographs of the pelvis. In cases with suspicious abnormalities (grade 1 of the New York criteria) CT permits the diagnosis since it shows a higher degree of sacroiliitis. MRI is superior to CT in the early diagnosis as it can detect the cartilage abnormalities which precede bony changes. 'Romanus lesions' with 'shiny corners', 'squaring' of the vertebral bodies, syndesmophytes, spondylodiscitis and osteoporosis are the radiological findings of ankylosing spondylitis. The nonmarginal, asymmetric, coarse and broad syndesmophytes of psoriatic spondylitis and spondylitis associated with Reiter's syndrome resemble the flowing bone outgrowths of diffuse idiopathic skeletal hyperostosis (DISH). The ossification of the posterior longitudinal ligament and of the flavum ligament are rare manifestations of ankylosing spondylitis. Peripheral extra-articular enthesitis is a clinical hallmark of seronegative spondylarthropathies. Plain film radiography shows erosions and spurs but only in advanced phases. US shows the swelling of the entheses and the peritendinous soft tissues and the distension of adjacent bursae by fluid collection. MRI shows the inflammation of the bone adjacent to the insertion as well as the soft tissue changes. Dactylitis is another typical manifestation of seronegative spondylarthropathies. In the past it was thought to be due to concomitant tenosynovitis and arthritis. Recently, however, we have demonstrated with US and MRI that the 'sausage-like' appearance is due to the flexor tenosynovitis and that joint capsule distension is not indispensable.