Macrophage activation syndrome (MAS) is a potentially life threatening complication of chronic rheumatic diseases, particularly systemic juvenile idiopathic arthritis (JIA). A number of triggers have been related to the development of MAS, including viral infections, nonsteroidal antiinflammatory drug therapy, and gold salt injections. We describe a patient with systemic JIA who developed MAS shortly after receiving methotrexate, suggesting that this drug can be regarded as a potential trigger of MAS in children with JIA.