OBJECTIVE Previous data suggests that patients with juvenile-onset ankylosing spondylitis (JoAS) have more severe disease and worse functional outcomes than adult-onset AS (AoAS). The purpose of this study was to evaluate clinical, functional and radiographic differences between patients with JoAS and AoAS in a large cohort of patients with long-standing disease. METHODS A total of 402 subjects who met the Modified New York Criteria for definitive AS and had had disease >or=20 years were enrolled in a multi-centre cross-sectional study (Prospective Study of Outcomes in Ankylosing Spondylitis; PSOAS). JoAS was defined as initial symptoms <or=16 years of age. A total of 79 subjects with JoAS and 323 subjects with AoAS were identified. An analysis of clinical and demographic comparisons between the two groups was performed including HLA B27 status. Functional outcomes were assessed by Bath AS Functional Index (BASFI) and the Health Assessment Questionnaire modified for the Spondyloarthropathies (HAQS). Radiographic disease severity was assessed by the Bath AS Radiology Index (BASRI). RESULTS With the exception of obvious differences in age at onset and disease duration, demographic and clinical characteristics were similar between the two groups. However, the JoAS group trended towards more women (32.9 vs 22.9%, p = 0.07). Controlling for multiple covariates including disease duration, both the BASRI hip score and the need for total hip arthroplasty (THA) was higher in the JoAS group. The BASRI spine score (including total, lumbar and cervical spine) was significantly lower in the patients with JoAS even after controlling for multiple covariates including disease duration and gender. No difference in function (BASFI or HAQS scores) between groups was identified. CONCLUSIONS Compared to AoAS, subjects with JoAS have (1) less severe axial involvement radiographically, (2) similar functional outcomes, (3) more hip involvement with a greater need for THA, and (4) a slightly higher proportion of women.