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. Rheumatic fever should be considered, especially in Maori and Pacific patients and check for evidence of streptococcal infection by serology and throat swab. If joint swelling is persistent beyond 3 weeks then referral to paediatric rheumatology is indicated.