Inclusion Criteria:

• Age ≥ 1 with a diagnosis of Still's Disease

• Active diseases defined as:

• CRP or ferritin levels greater than ULN, and any of:

  • Fever ≥ 38°C attributed to Still's Disease activity and documented for a number of days prior to Day 1 or
  • Rash attributed to Still's Disease activity or
  • Musculoskeletal involvement: arthritis in a number of joints per ACP criteria for active joint or
  • Serositis or
  • Macrophage activation syndrome activity as defined by ferritin levels and at least one of: platelet count, a biomarker or fibrinogen levels attributed to Still's Disease activity by the investigator

• Need for glucocorticoids (prednisone or equivalent)

Exclusion Criteria:

• Patients out of weight range

• Ongoing or previous treatment with immunomodulatory drugs

  • A limited number of Still's Disease patients that have previously received MAS825 through a managed access program are permitted on the study

• Glucocorticoid dose exceeding a set limit

• Any conditions or significant medical problems which places the patient at unacceptable risk for MAS825 therapy

  • Still's disease patients with evidence of macrophage activation syndrome are permitted in the study
  • Still's Disease patients with evidence of interstitial lung disease including those requiring supplemental oxygen therapy are permitted in the study

• History of ongoing, chronic or possibly recurrent infection (e.g. HIV, TB, HCV, HBV) and/or symptoms and signs of clinically significant active bacterial, fungal or viral infections

• Live vaccinations within a set time prior to MAS825 treatment. Live vaccines are prohibited up to several months following the last dose

• History of malignancy of any organ system, including post-transplant lymphoproliferative disorder, treated or untreated, within a number of years, regardless of whether there is evidence of local recurrence and metastases

• History of hypersensitivity to any of the study drugs or to drugs of similar chemical classes or to any of the excipients

• Pregnant or breastfeeding women

• Women of child-bearing potential who do not agree to comply with required contraceptive use

Other protocol-defined inclusion/exclusion criteria may apply.