One hundred and twenty-two consecutive hospitalized patients aged 35 years or more, and with definite radiographic bilateral sacro-iliitis were re-examined. All but 3 met established criteria for definite ankylosing spondylitis (AS). All peripheral joints from 114 of these patients were radiographed, and definite arthritis in one or several joints was demonstrated in 47 (41%) of them. The distribution pattern of arthritis was generally symmetric or slightly asymmetric. Articular new bone formation was a common finding in arthritic joints. The frequency of patients with hip arthritis was low (24% including patients with hip joint replacements); possible explanations include the selection of patients examined and difficulties in distinguishing between a primary osteoarthritis and osteoarthritis secondary to arthritis in a single radiographic examination in such patients. Although the hip joints were often severely affected, with extensive new bone formation, the erosions were generally small and protrusion of the acetabulum was not seen. Arthritis of the shoulder or of the hip was associated with an early onset of AS. Arthritis in the finger joints was associated with clinical psoriasis but was also a frequent finding in HLA B27-positive patients free from this condition (23%). Arthritic changes of small joints with extensive new bone formation ('egg in shell', 'pencil in cup' or 'gull wing' appearance) were seen in patients without clinical psoriasis. New bone formation of the ischium was frequent in HLA B27-positive patients (36%), but was not seen in HLA B27-negative patients. No differences concerning the frequency or severity of different radiographic pathologic changes recorded were observed between the sexes.