This study was performed to define the clinical spectrum and disease manifestations of ankylosing spondylitis (AS) in a referral hospital setting. We identified the differences in clinical manifestations according to the sex, the age at onset, the presence of peripheral arthritis and the presence of HLA B27. A total 412 patients (357 males, 55 females) were recruited. Eighty-seven percent were men and 155 out of 412 patients (35%) were juvenile-onset. HLA B27 was detected in 385 patients (93%). Peripheral joint involvement was noted in 287 of total AS cases (juvenile- onset ankylosing spondylitis (JOAS), 82%; adult-onset ankylosing spondylitis (AOAS), 61%), and was more common than those reported in other studies. A greater portion of patients with JOAS had peripheral arthritis and peripheral enthesitis than the patients with AOAS. The patients with peripheral arthritis showed a younger age at onset and an increased tendency of having enthesitis and trauma history. The natural history of Korean AS appears largely similar to those seen in Europe and North America, except a few differences. JOAS was quite common and AS was about nine times more common in men than in women. In addition, the HLA B27 antigen frequency was 93%, which is higher than those reported in other studies.