OBJECTIVE: Inflammatory arthritis in childhood is variable in terms of both presentation and outcome. This analysis describes disease activity in children with juvenile idiopathic arthritis (JIA) during the first year following presentation to a paediatric rheumatologist and identifies predictors of moderate to severe disability [defined using a Childhood HAQ (CHAQ) score >or=0.75] at 1 year. METHODS: The Childhood Arthritis Prospective Study recruits children <16 years with new inflammatory arthritis persisting for >or=2 weeks from five UK tertiary referral centres. Demographics, disease features, joint count, CHAQ, physician's global assessment, parent's general evaluation of well-being (PGE), ESR and treatment, are collected at first presentation, 6 months and then yearly. Independent predictors of CHAQ >or=0.75 at 1 year in children diagnosed with JIA were identified using multivariable logistic regression models. RESULTS: Seven hundred and forty children with JIA were included; median age at presentation 7.6 years, 64% girls. During the first year, 85% received NSAIDs, 70% IA corticosteroids, 47% MTX and 27% systemic steroids (oral or i.v.). Median presenting CHAQ score was 0.63 and decreased to 0.25 at 1 year; 32% had CHAQ >or=0.75 at 1 year. The strongest predictor of CHAQ >or=0.75 at 1 year was CHAQ >or=0.75 at presentation (odds ratio 3.92; 95% CI 2.17, 7.09). Additional predictors included female gender and higher PGE. CONCLUSION: Although CHAQ score improved in most children, the strongest predictor of persistent disability at 1 year was moderate to severe disability at first presentation. Follow-up beyond 1 year will assess whether CHAQ at presentation will continue to be a predictor of future poor outcome.