This study assessed the safety and preliminary efficacy of the interleukin-1 receptor antagonist anakinra in patients with polyarticular-course juvenile rheumatoid arthritis (JRA). Eighty-six patients entered a 12-week open-label run-in phase (1 mg/kg anakinra daily, < or =100 mg/day). Fifty responders were randomized to anakinra or placebo in a 16-week blinded phase, followed by a 12-month open-label extension (N = 44). Due to low enrollment, the primary endpoint was changed from efficacy to safety. The incidence and nature of adverse events were similar across all study phases, with the exception of injection site reactions, which were mild to moderate and decreased with time. Anakinra produced a nonsignificant (P = 0.11) reduction in disease flares compared with placebo. When normalized to 1 mg/kg dose, anakinra plasma concentrations were similar to values in adult patients with rheumatoid arthritis. These results indicate that anakinra 1 mg/kg once daily (< or =100 mg/day) is safe and well tolerated in patients with JRA.