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Objective To validate whether the 2016 American College of Rheumatology/European League Against Rheumatism classification criteria of macrophage activation syndrome (MAS) complicating systemic juvenile idiopathic arthritis (JIA) is practical in the real world. Methods A combination of expert consensus and analysis of real patient data was conducted by a panel of 15 pediatric rheumatologists. A total of 65 profiles comprised 18 patients with systemic JIA-associated MAS and 47 patients with active systemic JIA without evidence of MAS. From these profiles, 10 patient data points for full-blown MAS, 11 patient data points for MAS onset, and 47 patient data points for acute systemic JIA without MAS were evaluated. Results Evaluation of the classification criteria to discriminate full-blown MAS from acute systemic JIA without MAS showed a sensitivity of 1.000 and specificity of 1.000 at the time of full-blown MAS. Sensitivity was 0.636 and specificity was 1.000 at the time of MAS onset. The number of measurement items that fulfilled the criteria increased in full-blown MAS compared to that at MAS onset. At MAS onset, the positive rates of patients who met the criteria for platelet counts and triglycerides were low, whereas those for aspartate aminotransferase were relatively high. At full-blown MAS, the number of patients who met the criteria for each measurement item increased. Conclusion The classification criteria for MAS complicating systemic JIA had a very high diagnostic performance. However, the diagnostic sensitivity for MAS onset was relatively low. For the early diagnosis of MAS in systemic JIA, the dynamics of laboratory values during the course of MAS should be further investigated.