Patients with systemic onset juvenile idiopathic arthritis (SoJIA) are rarely known to develop coronary artery dilatation. The American heart association (AHA) statement on evaluation of suspected Kawasaki disease (KD) would lead some SoJIA patients (particularly in the early stages of the disease) to be inaccurately classified as KD. In addition to the institution of inappropriate therapy with IVIG, misdiagnosis as KD can delay definitive treatment for these SoJIA patients who probably have a worse predicted outcome. We present a 6-year-old male patient with SoJIA who was initially classified as incomplete KD. The child developed life-threatening macrophage activation syndrome (MAS). Previous literature regarding coronary dilatation in SoJIA is also reviewed.