Minoia F, Bovis F, Davì S, et al. - The clinical and laboratory features of 362 patients with systemic juvenile idiopathic arthritis (sJIA)-associated macrophage activation syndrome (MAS) and 404 patients with active sJIA without evidence of MAS were gathered in a multinational collaborative project in order to generate and verify a diagnostic score that helps in recognizing MAS in patients with sJIA. The MAS/sJIA (MS) score varies from −8.4 to 41.8 and includes seven variables, ie, central nervous system dysfunction, hemorrhagic manifestations, active arthritis, platelet count, fibrinogen, lactate dehydrogenase, and ferritin. A cut-off value ≥ −2.1 exhibited the best performance in distinguishing MAS from active sJIA, with a sensitivity of 0.85, a specificity of 0.95 and a kappa value of 0.80. The good performance of the MS score was verified in the validation sample. Thus, in patients with sJIA, the MS score is a robust and feasible tool that may aid practitioners in producing a timely diagnosis of MAS.