To investigate whether serum leucine-rich α2-glycoprotein (LRG) levels are useful as a marker of disease activity in systemic juvenile idiopathic arthritis (s-JIA), we determined serum LRG levels in fifty-nine s-JIA patients, 15 with other subtypes of JIA, 7 with Kawasaki disease (KD), 7 with influenza A infection (flu), 7 with enterohemorrhagic Escherichia coli (EHEC) infection, and 20 healthy controls (HC). Results were compared with the clinical features of s-JIA and serum cytokine levels including interleukin- (IL-) 6, IL-18, and soluble tumor necrosis factor receptors I and II. Serum LRG levels in active s-JIA were higher compared to those in other subtypes of JIA, EHEC, flu patients, and HC. Serum LRG levels were normalized in the inactive s-JIA phase after treatment. Serum LRG levels were positively correlated with serum C-reactive protein and ferritin levels. Serum LRG levels reflected s-JIA disease activity and thus may be useful for monitoring s-JIA disease activity.