You are being asked to take part in this research study because you have Systemic Juvenile Idiopathic Arthritis (sJIA) which may cause joint swelling and pain, fever, or other symptoms affecting the whole body (such as rash, swelling of your lymph nodes, a large liver or spleen, or inflammation of the lining around your lungs or heart). The purpose of this research study is to compare the effectiveness of the study drug, tofacitinib, to placebo for treatment of signs and symptoms of sJIA.