OBJECTIVE To compare functional outcome of patients with juvenile-onset ankylosing spondylitis (JoAS; defined as AS with symptom onset before 16 years of age) with that of patients with adult-onset AS (AoAS) and to identify variables associated with a poor functional outcome of JoAS. METHODS A cross-sectional study was performed of 326 JoAS patients who participated in a postal survey conducted by the Spondylitis Association of America. This cohort was compared with 2,021 AoAS patients who participated in the same survey. Simple and multiple logistic regression analyses were performed to identify differences with respect to clinical features, demographic features, and functional outcome (defined by the Bath Ankylosing Spondylitis Functional Index [BASFI]) between the 2 groups. A validation cohort of 255 AS patients was also surveyed. RESULTS The mean +/- SD BASFI score (controlled for disease duration) for JoAS was 51.3 +/- 1.5 compared with 46.4 +/- 0.6 for AoAS (P < 0.0001). Multiple logistic regression identified only age (P < 0.0001) and income status (P < 0.0001) as factors associated with functional impairment. CONCLUSIONS It appears that JoAS is a progressive disease and is associated with significant delay in diagnosis and worse functional outcome compared with AoAS. Furthermore, women do worse than men with JoAS. This would argue for the importance of early diagnosis and treatment of AS, particularly in the subgroup of patients with JoAS.