Objective This study evaluated the efficacy and safety of baricitinib,an oral Janus kinase (JAK) I/JAK2 inhibitor,in patients with moderately to severely active rheumatoid arthritis(RA) and inadequate response to methotrexate (MTX) therapy. Methods In this phase 3, double-blind, 52-week, placebo-controlled study, 290 patients with moderately to severely active RA and inadequate response to MTX were randomly assigned 1:1 to placebo or baricitinib 4-mg once daily, stratified by country (China,Brazil, Argentina) and presence of joint erosions. Primary endpoint measures included American College of Rheumatology 20% response (ACR20) at week 12. Secondary endpoints included changes in Health Assessment Questionnaire-Disability Index (HAQ-DI) and Disease Activity Score for 28-joint counts (DAS28)-high-sensitivity C-reactive protein (hsCRP), Simplified Disease Activity Index (SDAI) score <= 3.3, mean duration of morning joint stiffness, severity of morning joint stiffness numeric rating scale (NRS 0-10), worst tiredness NRS, and worst joint pain NRS at week 12. Results Most patients (approximately 80%) were from China. More patients achieved ACR20 response at week 12 with baricitinib than with placebo (58.6% vs. 28.3%; p<0.001). Statistically significant improvements were also seen in HAQ-DI, DAS28-hsCRP, morning joint stiffness, worst tiredness, and worst joint pain in the baricitinib group compared to placebo at week 12. Through week 24, rates of treatment-emergent adverse events, including infections, were higher for baricitinib compared to placebo, while serious adverse event rates were similar between baricitinib and placebo. Conclusion In patients with RA who had an inadequate response to MTX, baricitinib was associated with significant clinical improvements as compared with placebo.