To compare the efficacy, safety, and tolerability of 5 doses of oral tofacitinib (CP-690,550) or adalimumab monotherapy with placebo for the treatment of active rheumatoid arthritis (RA) in patients with an inadequate response to disease-modifying antirheumatic drugs. In this 24-week, double-blind, phase IIb study, patients with RA (n = 384) were randomized to receive placebo, tofacitinib at 1, 3, 5, 10, or 15 mg administered orally twice a day, or adalimumab at 40 mg injected subcutaneously every 2 weeks (total of 6 injections) followed by oral tofacitinib at 5 mg twice a day for 12 weeks. The primary end point was the responder rate according to the American College of Rheumatology 20% improvement criteria (ACR20) at week 12. Treatment with tofacitinib at a dose of ≥3 mg twice a day resulted in a rapid response with significant efficacy when compared to placebo, as indicated by the primary end point (ACR20 response at week 12), achieved in 39.2% (3 mg; P ≤ 0.05), 59.2% (5 mg; P < 0.0001), 70.5% (10 mg; P < 0.0001), and 71.9% (15 mg; P < 0.0001) in the tofacitinib group and 35.9% of patients in the adalimumab group (P = 0.105), compared with 22.0% of patients receiving placebo. Improvements were sustained at week 24, according to the ACR20, ACR50, and ACR70 response rates as well as classifications of remission according to the 3-variable Disease Activity Score in 28 joints (DAS28) using C-reactive protein and the 4-variable DAS28 using the erythrocyte sedimentation rate. The most common treatment-emergent adverse events (AEs) in patients across all tofacitinib treatment arms (n = 272) were urinary tract infection (7.7%), diarrhea (4.8%), headache (4.8%), and bronchitis (4.8%). Tofacitinib monotherapy at ≥3 mg twice a day was efficacious in the treatment of patients with active RA over 24 weeks and demonstrated a manageable safety profile. Copyright © 2012 by the American College of Rheumatology.