JIA patients aged 4 to 8 years and treated by the pediatric rheumatology units from various University Medical Centers in the Netherlands, are asked to participate in a prospective study. In the Netherlands, measles-mumps-rubella (MMR) vaccination is included in the National Vaccination Program and is normally administered at age 9. Included patients will be randomised for early vaccination (age group 4 to 8yr at entry of the study) or at age 9 as is routinely done according to the National Vaccination Program. Prior to and after vaccination the investigators will assess disease activity and collect blood.