The reason for this study is to see if the study drug baricitinib is safe and effective in participants from 1 year to less than 18 years old with systemic juvenile idiopathic arthritis (sJIA). Participants are assigned to 1 of 2 cohorts. In cohort 1, participants will receive baricitinib or tocilizumab reference. In cohort 2, participants will receive baricitinib.

Inclusion Criteria:

• Participants must have a diagnosis of systemic Juvenile Idiopathic Arthritis (sJIA) as defined by International League of Associations for Rheumatology (ILAR) criteria with onset before the age of 16 years
• Participants must have at least 2 active joints at screening and baseline
• Cohort 1 (IL-6 inhibitor therapy naive): Participants who are at least 1 year and less than 18 years of age, except in countries that restrict use of tocilizumab in participants less than 2 years of age
• Cohort 2 (open-label baricitinib): Participants who are at least 1 year and less than 18 years of age

Exclusion Criteria:

• Participants must not have polyarticular JIA (positive or negative for rheumatoid factor), extended oligoarticular JIA, enthesitis-related JIA, or juvenile psoriatic arthritis
• Participants must not have persistent oligoarticular arthritis as defined by the ILAR criteria
• Participants must not have a history or presence of any autoimmune inflammatory condition other than JIA
• Participants must not have active anterior uveitis or are receiving concurrent treatment for anterior uveitis
• Participants must not have active fibromyalgia or other chronic pain conditions that, in the investigator's opinion, would make it difficult to appropriately assess disease activity for the purposes of this study
• Participants must not have biologic features of Macrophage Activation Syndrome (MAS) over the past 12 weeks
• Participants must not have a current or recent (<4 weeks prior to baseline) clinically serious infection
• Participants must not have a positive test for hepatitis B virus
• Participants must not have evidence of active tuberculosis (TB) or untreated latent TB

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and EU, whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.