Assessment of disease activity in children with juvenile idiopathic arthritis (JIA) is central to avoiding permanent joint damage. Affected joints are characterised by synovial proliferation resulting in synovial effusion and hypertrophy. Long-standing poorly controlled synovitis is associated with a high frequency of joint damage. 11 children with JIA were examined with quantitative MRI to measure synovial volume and rate of contrast uptake before and 12 weeks after routine intra-articular corticosteroid injection. All children showed marked reductions in synovial volumes, and in rates of uptake of gadolinium. These reductions were more consistent than changes in clinical assessments.