Study of Human Anti-TNF Monoclonal Antibody Adalimumab in Children With Polyarticular Juvenile Idiopathic Arthritis (JIA)

- Subjects must have a diagnosis of polyarticular juvenile idiopathic arthritis (JIA) age 4 to 17 by the American College of Rheumatology (ACR) criteria. Disease onset may have been systemic, polyarticular, or pauciarticular. If the disease was systemic onset, then the subjects must be free of any systemic JIA manifestations for at least 3 months before the time of qualification. - At the time of study screening, the subject must have continuing active disease defined as >= 5 swollen joints and >= 3 joints with limitation of motion (LOM). These joints are not mutually exclusive. - Subjects may be either naïve to MTX, inadequate responders to MTX, or intolerant to MTX. Intolerance to MTX will be defined by the subject's physician. The MTX must be maintained at a dose of at least 10 mg/m2 body surface area/week for a minimum of 3 months, prior to screening. - Duration of disease is not limited, but must have been long enough for a subject to have been given an adequate trial of nonsteroidal anti-inflammatory drugs (NSAIDs). - Have not received other disease-modifying anti-rheumatic drugs (DMARDs) including penicillamine, hydroxychloroquine, sulfasalazine, oral or injectable gold, cyclosporin; or intravenous immunoglobulin (IV Ig); or cytotoxic agents, for at least 4 weeks prior to receiving 1st dose of study drug. Subjects currently on one or more of these DMARDs must demonstrate active disease (defined above) prior to a minimum 4 weeks (28 days) washout of all DMARDs. - Subjects who are refractory to MTX after 3 months of treatment must demonstrate active disease (defined above) prior to enrollment in the open-label part of the trial. - Have not received an intra-articular glucocorticoid injection within 4 weeks (28 days) prior to enrollment into the study. - Have good venous access and stable hematocrit >= 24%. - All sexually active male and female study participants must be practicing adequate contraception. Post-pubertal females must have a negative serum pregnancy test no greater than 10 days prior to the first dose of study drug. - Parent or guardian has voluntarily signed and dated an informed consent form, approved by an Institutional Review Board (IRB)/Independent Ethics Committee (IEC), after the nature of the study has been explained and the subject's parent or legal guardian has had the opportunity to ask questions.

- Pregnant or nursing female. - Functional class IV by ACR criteria. - Laboratory parameters outside limits established in the protocol. - Medical history, medical condition, or previous treatment not allowed by the protocol.