Kawasaki Disease (KD) is one of the most common vasculitides in childhood and represents a leading cause of acquired heart disease in developed countries. Macrophage Activation Syndrome (MAS) is a potentially life threatening hyperinflammatory condition belonging to the spectrum of hemophagocytic lymphohistiocytosis (HLH), and it can complicate various rheumatologic diseases. Awareness of MAS in the context of KD has recently increased, supporting the hypothesis that it is an underdiagnosed complication. The study aims to define the epidemiology, clinical characteristics, management, and therapeutic strategies of MAS in patients with KD, through a multicenter data collection in Europe.