ObjectivesTo validate the correlation between laboratory markers reflecting disease activity of macrophage activation syndrome (MAS) and serum cytokine levels and identify the valuable laboratory markers that change over time for a prompt MAS diagnosis. MethodsSerum cytokine levels were determined by enzyme-linked immunosorbent assay and compared with laboratory markers reflecting MAS disease activity.The changes in values were evaluated from the acute phase of systemic juvenile idiopathic arthritis (s-JIA) to MAS diagnosis. ResultsCXCL9 was significantly correlated with aspartate aminotransferase (AST), lactate dehydrogenase (LDH), D dimer, and urine β2 microglobulin levels. sTNF-RII was significantly correlated with platelet counts, AST, LDH, D dimer, and ferritin levels. Significant changes in platelet count, LDH, and D dimer levels were observed. Decreased platelet counts were the most valuable indicator for MAS diagnosis. ConclusionMonitoring the laboratory markers that change over time, particularly decreased platelet counts, was valuable for the prompt MAS diagnosis in s-JIA.