Changes in T cell homoeostasis, differentiation and function have been described in circulation and locally in the joints.46–53 Particularly, disturbances in Tregs were observed in JIA.54 Accordingly, we found that children with extended oligo JIA and poly JIA have lower levels of Tregs (CD4+CD25+FoxP3+) in circulation when compared with controls, suggesting that a breakdown of Treg-mediated peripheral tolerance might occur in JIA pathogenesis. Here, FOXP3 is linked to juvenile idiopathic arthritis.