To evaluate the applicability of the 2016 European Alliance of Associations for Rheumatology (EULAR)/American College of Rheumatology (ACR)/Paediatric Rheumatology International Trials Organisation (PRINTO) macrophage activation syndrome (MAS) classification criteria in patients with Still disease and systemic juvenile idiopathic arthritis (sJIA‐SD) treated with interleukin‐1 (IL‐1)–targeted therapy and to assess the incidence of MAS in this context. Here, IL1B is linked to systemic-onset juvenile idiopathic arthritis.