As part of its single technology appraisal process, the National Institute for Health and Care Excellence invited the manufacturer (Eli Lilly) of baricitinib (BARI; Olumiant®; a Janus kinase inhibitor that is taken orally) to submit evidence of its clinical and cost effectiveness for the treatment of moderate to severe rheumatoid arthritis (RA) after the failure of disease-modifying antirheumatic drugs (DMARDs). The School of Health and Related Research Technology Appraisal Group at the University of Sheffield was commissioned to act as the independent Evidence Review Group (ERG). The ERG produced a detailed review of the evidence for the clinical and cost effectiveness of the technology, based on the company’s submission (CS) to NICE. The clinical-effectiveness evidence in the CS for BARI was based predominantly on three randomised controlled trials comparing the efficacy of BARI against adalimumab or placebo, as well as one long-term extension study. The clinical-effectiveness review identified no head-to-head evidence on the efficacy of BARI against all the comparators within the scope. Therefore, the company performed network meta-analyses (NMAs) in two different populations: one in patients who had experienced an inadequate response to conventional DMARDs (cDMARD-IR), and the other in patients who had experienced an inadequate response to tumour necrosis factor inhibitors (TNFi-IR). The company’s NMAs concluded BARI had comparable efficacy as the majority of its comparators in both populations. The company submitted a de novo discrete event simulation model that analysed the incremental cost-effectiveness of BARI versus its comparators for the treatment of RA from the perspective of the National Health Service (NHS) in four different populations: (1) cDMARD-IR patients with moderate RA, defined as a 28-Joint Disease Activity Score (DAS28) > 3.2 and no more than 5.1; (2) cDMARD-IR patients with severe RA (defined as a DAS28 > 5.1); (3) TNFi-IR patients with severe RA for whom rituximab (RTX) was eligible; and (4) TNFi-IR patients with severe RA for whom RTX in combination with methotrexate (MTX) is contraindicated or not tolerated. In the cDMARD-IR population with moderate RA, the deterministic incremental cost-effectiveness ratio (ICER) for BARI in combination with MTX compared with intensive cDMARDs was estimated to be £37,420 per quality-adjusted life-year (QALY) gained. In the cDMARD-IR population with severe RA, BARI in combination with MTX dominated all comparators except for certolizumab pegol (CTZ) in combination with MTX, with the ICER of CTZ in combination with MTX compared with BARI in combination with MTX estimated to be £18,400 per QALY gained. In the TNFi-IR population with severe RA, when RTX in combination with MTX was an option, BARI in combination with MTX was dominated by RTX in combination with MTX. In the TNFi-IR population with severe RA for whom RTX in combination with MTX is contraindicated or not tolerated, BARI in combination with MTX dominated golimumab in combination with MTX and was less effective and less expensive than the remaining comparators. Following a critique of the model, the ERG undertook exploratory analyses after applying corrections to the methods used in the NMAs and two programming errors in the economic model that affected the company’s probabilistic sensitivity analysis (PSA) results. The ERG’s NMA results were broadly comparable with the company’s results. The programming error that affected the PSA of the severe cDMARD-IR population had only a minimal impact on the results, while the error affecting the severe TNFi-IR RTX-ineligible population resulted in markedly higher costs and QALYs gained for the affected comparators but did not substantially modify the conclusions of the analysis. The NICE Appraisal Committee concluded that BARI in combination with MTX or as monotherapy is a cost-effective use of NHS resources in patients with severe RA, except in TNFi-IR patients who are RTX-eligible.