Reactive arthritis follows infection, which may be mild (e.g. upper respiratory tract infection), or more obvious (tonsillitis, gastroenteritis). In the adolescent consider sexually acquired infection. Limp may be present. Reactive arthritis will usually settle within a few weeks. Septic arthritis or osteomyelitis needs to be considered. If joint swelling is persistent beyond 3 weeks then referral to paediatric rheumatology is indicated.