Ankylosing spondylitis is an inflammatory disorder of unknown etiology that primarily affect, the spine, the axial skeleton, and the large proximal joints of the body with striking tendancy toward fibrosis with secondary ossification and ankylosis of involved joints. This study was carried out to analyse clinical manifestation, evaluate disease activity indicator and set up diagnostic work-up. 38 cases of ankylosing spondylitis were studied at Department of Orthopedic Surgery, Seoul National University from Oct. 1987 to Mar. 1989. Following results were obtained. l. Average age was 28.5 years and all patients were male. 2. Low back pain was the most frequent symptom(63%) and back stiffness was next(13%). 3. S-I joint change on x-ray was the most frequently found abnormalities(100%) and HLA B27 (+) was next(97%). 4. SI/S ratio was inversely correlated with duration of symptom and seems to be significant durng the early phase of disease. 5. As a disease activity indicator, CRP was more reliable than ESR during follow up.