The study design for this clinical trial was chosen to evaluate adalimumab in subjects who were either methotrexate (MTX)-naive or had been withdrawn from MTX at least 2 weeks prior to study drug administration (non-MTX stratum) or were inadequate responders to MTX and continued MTX treatment (MTX stratum). The study consisted of 4 phases: a 16-week Open-label Lead-in (OL LI), a 32-week Double-blind (DB) phase, an up to 136-week Open-label Extension Body Surface Area (OLE BSA) phase, and an up to 224-week OLE Fixed Dose (FD) phase. All subjects who met entry criteria were enrolled into one of the appropriate strata and received adalimumab (plus concomitant MTX in the MTX stratum) in the 16 week OL LI phase of the study. All subjects who responded to adalimumab during the OL LI phase were to be enrolled in the DB phase of the study and randomized to receive adalimumab (plus concomitant MTX in the MTX stratum) or placebo (plus concomitant MTX in the MTX stratum). Subjects in the DB phase received either adalimumab (24 mg/m2 BSA up to a maximum of 40 mg total body dose) or placebo subcutaneously (SC) administered every other week (eow). Adalimumab or placebo was administered for an additional 32 weeks or until flare of disease (based on PedACR30 response criteria = a worsening of 30% or more in 3 of the 6 response variables (Parent's global assessment of subject's overall well-being by visual analog scale [VAS], Physician's global assessment [PhGA] of subject's disease severity by VAS, number of active joints [joints with swelling not due to deformity or joints with limitation of passive motion (LOM)], pain, tenderness, or both, number of joints with LOM, Childhood Health Assessment Questionnaire [CHAQ], and CRP levels), a minimum of 2 active joints, and no more than 1 indicator improving by 30% or more), whichever occurred earlier. For subjects who did not have a disease flare, the DB phase was completed at Week 48. Subjects who experienced disease flare during the DB phase or subjects who completed 48 weeks of the study were given the option to receive adalimumab for up to a minimum of 44 weeks (up to a maximum of 136 weeks) in the OLE BSA phase before being eligible to switch to the OLE FD phase. In this phase, subjects received OL adalimumab (24 mg/m2 BSA up to a maximum of 40 mg total body dose SC eow). All subjects who completed at least 44 weeks of OLE BSA treatment were given the opportunity to continue into the OLE FD phase for up to 224 weeks of additional adalimumab exposure. In this phase, subjects weighing less than 30 kg were treated with a fixed dose of 20 mg of adalimumab SC eow. Subjects weighing 30 kg or more were treated with a fixed dose of 40 mg of adalimumab SC eow.