Macrophage activation syndrome (MAS) or hemophagocytic syndrome is a severe complication of chronic rheumatic diseases especially in systemic-onset juvenile rheumatoid arthritis (JRA). Although the cause of MAS is unknown, dysregulation of macrophage-lymphocyte interactions with subsequent increases in the levels of both T cell-derived and macrophage-derived cytokines could be involved in this syndrome, leading to an intense systemic inflammatory reaction, which accounts for the main clinical picture. Patients usually present with an acute febrile illness, hepatosplenomegaly, lymphadenopathy, cutaneous and mucosal bleeding, pancytopenia, and central nervous system, cardiac, and renal involvement. Treatment of MAS in patients with rheumatic diseases has not been standardized yet, but it commonly includes a variety of agents such as high-dose corticosteroids, cyclosporine, cyclophosphamide, etoposide, and intravenous immunoglobulin (IVIG). This article reviews the current literature about the pathogenesis, clinical manifestation, diagnosis, and treatment of this severe complication associated with rheumatic diseases.