Background Macrophage activation syndrome (MAS) is a rare,life-threatening complication of Kawasaki disease (KD).Early recognition and treatment of MAS are very important,but sometimes it is difficult to distinguish MAS from a severe form of KD.Data sources A PubMed search was performed in Clinical Queries using the key terms “macrophage activation syndrome or secondary hemophagocytic lymphohistiocytosis (HLH)” and “Kawasaki disease”.Results KD patients with MAS show high intravenous immunoglobulin (IVIG) resistance and coronary complications.Mortality is also as high as MAS in other diseases.Persistent fever greater than 10 days is highly associated with develop?ment of MAS in KD.Splenomegaly is observed in more than two-thirds of KD patients with MAS.Thrombocytopenia is often the earliest laboratory finding of MAS.Hyperferritinemia is highly specific and sensitive for detecting MAS in KD;so,ferritin levels should be checked if there are unexplained clinical exacerbations in KD patients.Given the under-recognition of MAS in KD,it is prudent to consider resistant KD as occult/subclinical MAS.Many KD patients with MAS have good outcomes on immune modulators.However,if KD patients fulfill the HLH-2004 diagnostic criteria,they may undergo longer and more intensive treatment than needed.Conclusions The possible existence of MAS should be taken into account when a KD patient shows persistent fever,sple?nomegaly,thrombocytopenia,hyperferritinemia,or IVIG resistance.The under-diagnosis of MAS in patients with KD is an important issue to be addressed.Therapeutically,however,there is a possibility of over-treatment of MAS in patients with KD.