Systemic juvenile idiopathic arthritis (sJIA) is a disease characterized by arthritis in children <16 years of age. JIA occurs in approximately 11/100,000 children annually in Europe and the United States, with systemic onset in 6–11%.1,2 sJIA is associated with serious complications including growth failure, osteoporosis, and musculoskeletal deformities; the main causes of sJIA-related deaths are infection and macrophage activation syndrome.3–5 Because interleukin-1 (IL-1) is a key mediator of the inflammatory cascade contributing to sJIA pathogenesis,6,7 this pro-inflammatory cytokine represents a therapeutic target. Rilonacept (IL-1 Trap) is an IL-1 neutralizer incorporating into 1 molecule the extra-cellular domain of 2 human cytokine receptors required for IL-1 signaling, combined with the Fc portion of human IgG1.8 Rilonacept binds to IL-1β with high affinity and specificity, and blocks inflammation caused by overproduction of IL-1. This novel therapeutic molecule is therefore hypothesized to be effective against auto-immune diseases such as sJIA. The pharmacokinetics (PK) of rilonacept have been evaluated in adults and children in a small study. In adults, the terminal apparent elimination half-life (T1/2) was approximately 1 week (154–184 hours) irrespective of dose, route of administration, or disease status.8 In children, a mean apparent T1/2 of 151 hours was observed.8 Rilonacept PK is not affected by renal failure as evidenced by similar PK observed in patients with end-stage renal disease on hemodialysis.9 Three clinical trials evaluating rilonacept in adults with rheumatoid arthritis have been completed: 2 phase I studies (combined N=137 patients) and 1 phase II trial (N=201). In both, rilonacept was generally well tolerated. The most frequently reported adverse event was injection site reaction, and no serious adverse events were felt to be drug-related. These studies suggested the best dose for children is 2–4 mg/kg/week subcutaneously (SC). Recently, the Randomized, Placebo Phase Study of Rilonacept in sJIA (RAPPORT) study evaluated the efficacy and safety of rilonacept in children with sJIA.10 Dosing data in this trial were informed by review of data from the Regeneron pilot study, where there was no difference in efficacy in children who received a 4.4mg/kg loading dose/2.2mg/kg maintenance dose vs a loading dose of 8.8mg/kg and a maintenance dose of 4.4mg/kg.11 The current report presents the PK analysis of rilonacept conducted in this study.