Galectin 9 and CXC-chemokine ligand 10 (CXCL10) have been validated as serum biomarkers of disease activity in a group of 125 patients with juvenile dermatomyositis (JDM) from three international cohorts. The combination of high concentrations of galectin 9 and CXCL10 was better able to differentiate patients with JDM with active disease from those in remission (area under curve (AUC) 0.86–0.90) than the standard marker creatine kinase (AUC 0.66–0.68), and could also predict flare several months before onset. Galectin 9 and CXCL10 were also predictive of active disease in adults with dermatomyositis.