Assessment of Macrophage Activation syndromE in STill's Disease

Inclusion Criteria: * Age \>6 months and ≤80 years at the beginning of the index MAS episode. * Diagnosis of Still's disease (sJIA (systemic Juvenile Idiopathic Arthritis) or AOSD (Adult Onset Stills Disease) diagnosis). * Diagnosis of MAS according to treating physician in the medical record. * Patients who have received at least 3 consecutive days of GC after diagnosis of MAS and/or are judged by the Investigator to be refractory to GC due to clinical worsening of patient's condition . * The onset of the index MAS episode occurred between 01 January 2012 and 30 September 2022. Exclusion Criteria: * A diagnosis of primary Hemophagocytic Lymphohistiocytosis (HLH) prior to the beginning of the index MAS episode. * Confirmed malignancy prior to the beginning of the index MAS episode. * Patient treated with any investigational product as a part of clinical trial during the index MAS episode.