The safety of vaccination in patients with autoimmune diseases using immune suppressive therapy is often discussed. Previous studies in Juvenile Idiopathic Arthritis (JIA) patients showed no increase in disease activity after immunisation with dead vaccines. The safety of the live attenuated Measles, Mumps, Rubella (MMR) vaccination was assessed retrospectively in JIA patients and no increase in disease activity was found. However, this must be prospectively confirmed. In addition, it is unknown whether vaccination is effective, since the immune response to vaccination may be diminished due to immunosuppressive therapy for the underlying disease. Finally, the influence of MMR vaccination on the immune system of JIA patients has not been studied. Among others, regulatory T-cells (Tregs) should control the immune response and prevent destructive autoimmune responses after environmental triggers such as vaccination.