BACKGROUND Ankylosing spondylitis (AS) is a chronic inflammatory disease, which involves the spine, peripheral joints and entheses. Juvenile-onset ankylosing spondylitis (JAS) affects children under the age of 16 years. JAS has been noted to present as clinical courses different from those of adult-onset ankylosing spondylitis (AAS). Therefore, the purpose of the present study was to compare the possible risk factors, clinical manifestations, laboratory markers, radiological changes, and functional outcome between these 2 patient groups. METHODS AS patients were enrolled from the rheumatologic clinic of a tertiary medical center from January 1 to June 30 in 2006. The demographic data, clinical symptoms/signs, Bath AS indices, HLA-B27, inflammatory markers, radiological findings, and treatment history were acquired with questionnaires, clinical evaluation, and chart review. The differences between JAS and AAS patients were evaluated and analyzed. RESULTS A total of 169 patients (142 males, 27 females) were included, comprising 47 JAS and 122 AAS patients. The ages of onset were 12.8 +/- 2.7 years and 25.0 +/- 7.4 years for JAS and AAS, respectively. They had similar gender distribution, years of delay to diagnosis and disease duration. A substantial proportion of our patients (40.4% of JAS and 34.4% of AAS) had physical trauma in the 1 month before disease onset. Also, 22.7% of JAS patients had intense physical training, while 25.2% of AAS patients did heavy work during the period. The first manifestation of JAS was mainly peripheral enthesopathy or arthritis, but axial symptoms in most AAS. More JAS patients had peripheral enthesopathies and arthritis on any occasion. Although there was a trend of higher score in Bath AS Disease Activity Index (BASDAI), Bath AS Metrology Index (BASMI) and Physician's Global Assessment (PGA) score, JAS patients had a comparable Bath AS Functional Index (BASFI) and Bath AS Patient's Global Assessment (BAS-G) as AAS patients. As to the laboratory and radiological tests, JAS patients had higher levels of C-reactive protein and erythrocyte sedimentation rate, and more radiographic changes of hip joints. CONCLUSIONS JAS and AAS patients had distinct presentations. JAS presented more peripheral enthesopathies and arthritis at disease onset and at any time of the course. If treated effectively, JAS will not lead to a worse functional outcome than AAS. Therefore, it is mandatory to diagnose and treat JAS as early as possible.