Adult onset Still's disease (AOSD) is a rare systemic inflammatory disorder of unknown etiology. Systemic onset juvenile idiopathic arthritis (SoJIA) is the preferred nomenclature of Still's disease. Strong association with so-called macrophage activation syndrome (MAS) may provide a clue to the understanding of the distinctive pathogenetic features of SoJIA. MAS is a severe, potentially life-threatening complication characterized by the excessive activation of well-differentiated macrophages. It is more appropriately named autoimmune disease associated reactive hemophagocytic lymphohistiocytosis (ReHLH), a subset of a histiocytic disorder: class II histiocytosis hemophagocytic lymphohistiocytosis (HLH). The relation of SoJIA with HLH is still under debate. We propose that MAS, HLH, SoJIA, and AOSD are indeed the same disease, in different clinical presentations that may be classified based on severity and laboratory findings, but with essentially the same physiopathogenesis. We propose that the case described by Hong & Lee (Rheumatol Int 2008) was actually an AOSD-associated MAS/RHS/ReHLH fulminant disease.