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Early diagnosis of systemic juvenile idiopathic arthritis (s‐JIA) is a prerequisite for therapeutic efficacy. However, it is often challenging because most patients with s‐JIA do not show arthritis at disease onset and are simply diagnosed with fever of unknown origin. Serum ferritin levels have commonly been used to diagnose s‐JIA because they increase in patients with this condition by more than 5 times their normal value. However, there are no definite biomarkers for s‐JIA, which makes the clinical diagnosis of s‐JIA difficult. We report a case of s‐JIA in which interleukin (IL)‐18 elevation was observed before ferritin elevation at the early phase of s‐JIA. We propose serum IL‐18 levels as a more useful biomarker for the early diagnosis of s‐JIA compared to serum ferritin levels. [ABSTRACT FROM AUTHOR]