Background: Biologic disease modifying anti-rheumatic drugs (bDMARDs) are recommended for radiographic axial spondyloarthritis (r-axSpA), otherwise known as ankylosing spondylitis, when conventional therapies fail. We report efficacy and safety results of a Phase 3 study of ixekizumab, a high-affinity monoclonal antibody that selectively targets IL-17A, in bDMARDnaïve patients with r-axSpA. Methods: In this randomized, double-blind, Phase 3 study, adult patients with inadequate response/intolerance to NSAIDs, an established diagnosis of r-axSpA, and with radiographic sacroiliitis centrally defined by modified New York criteria and ≥1 spondyloarthritis feature according to Assessment of Spondyloarthritis International Society (ASAS) criteria were recruited from 84 sites (12 countries) in Europe, Asia, and North America. Patients were randomized 1:1:1:1 using a computer-generated random sequence to 80 mg subcutaneous ixekizumab every two (Q2W) or four (Q4W) weeks, 40 mg adalimumab Q2W (active reference arm), or placebo. The primary endpoint was the proportion of patients achieving an ASAS40 response at Week 16. Findings: Between June 20, 2016 and August 22, 2017, 341 patients were randomized to placebo (N=87), adalimumab (N=90), ixekizumab Q2W (N=83), or ixekizumab Q4W (N=81). At Week 16, significantly more patients achieved ASAS40 with ixekizumab Q2W (n=43, 51•8%, p<0•0001), ixekizumab Q4W (n=39, 48•1%, p<0•0001), and adalimumab (n=32, 35•6%; p=0•0053) versus placebo (n=16, 18•4%). One serious infection occurred in each of the ixekizumab Q2W (1•2%), ixekizumab Q4W (1•2%), and adalimumab (1•1%) arms; none were reported with placebo. One (1•1%) Candida infection occurred in the adalimumab arm and one (1•2%) patient receiving ixekizumab Q2W was adjudicated as having probable Crohn's disease. No opportunistic infections, malignancies, or deaths occurred. Interpretation: Each dosing regimen of ixekizumab was superior to placebo for improving r-axSpA signs and symptoms in bDMARD-naïve patients; the safety profile was consistent with previous studies of ixekizumab. The adalimumab control arm performed as expected. Funding: Eli Lilly and Company Research in context Evidence before this study Pubmed was searched using the terms "ankylosing spondylitis", "axial spondyloarthritis", and "disease-modifying anti-rheumatic drugs", including articles through May 30, 2018. Axial spondyloarthritis (axSpA) is a chronic immune-mediated disease characterized by inflammation of the spine and sacroiliac joint (SIJ), peripheral joint involvement, extra articular manifestations, and a strong genetic association with human leukocyte antigen (HLA)-B27. Radiographic axSpA (r-axSpA) was previously classified as ankylosing spondylitis (AS) in 1984 and updated to r-axSpA as part of the ASAS criteria. Both criteria sets require the same radiographically confirmed structural damage to the sacroiliac joint as well as at least one accompanying clinical element. Recommendations for the management of r-axSpA generally include exercise and physiothera...