OBJECTIVE: Ankylosing spondylitis is a well known form of spondyloarthropathy. It is recognized that age at onset is factor that may influence both the clinical presentation and course of disease. The pattern of symptoms in juvenile onset ankylosing spondylitis (JAS) differs significantly from that of adult onset (AS). METHODS: We investigated retrospectively the clinical characteristics of Korean JAS and AS. We reviewed two hundred and forty-two patients who had met the Modified New York criteria for AS. RESULTS: The JAS:AS ratio in the total AS patients was 88:154. The age of onset was 12.9+/-.67 (mean+/-D) in JAS and 22.6+/-.35 in AS. The sex ratio (M:F 81:7 in JAS, M:F 127:27 in AS) and disease duration (9.7+/-.7 in JAS, 801+/-.8 in AS) were similar in both groups. The most common site of initial symptom was knee in both groups, more frequent in JAS than in AS (JAS: 38%, AS: 25%, p<0.05). The peripheral arthritis was more frequent than axial symptom in initial symptom of both groups (JAS: 84%, AS:65%, p<0.01). There were significant differences in knee arthritis (JAS: 83%, AS:67%, P<0.001), ankle arthritis (JAS: 63%, AS: 39%, p<0.001), and tarsus involvement (JAS: 32%, AS: 19% P<0.05). The peripheral arthritis was found in 79%, more frequent in JAS than in AS (JAS: 90%, AS: 73%, p<0.001),. The arthritis in lower extremities was more frequent in JAS than in AS (JAS: 90%, AS: 72%, p<0.001). There were no differences in laboratory findings, spine involvement, and extraarticular symptoms including renal involvement, uveitis, and enthesitis (JAS: 86%, AS: 81%) between both groups. CONCLUSIONS: Contrary to foreign reports, the peripheral arthritis was more common, sex ratio of male versus female was higher among JAS in Korea. The peripheral arthritis in JAS is more frequent than in AS, and main symptoms in JAS was found more common in peripheral than in axial joints. There were significant differences on initial symptom site and peripheral arthritis of lower extremity, especially knee, ankle and tarsus in both group.