Objective: To assess the efficacy/safety of tofacitinib in adult patients with active ankylosing spondylitis (AS)., Methods: This phase III, randomised, double-blind, placebo-controlled study enrolled patients aged ≥18 years diagnosed with active AS, meeting the modified New York criteria, with centrally read radiographs, and an inadequate response or intolerance to ≥2 non-steroidal anti-inflammatory drugs. Patients were randomised 1:1 to receive tofacitinib 5 mg two times per day or placebo for 16 weeks. After week 16, all patients received open-label tofacitinib until week 48. The primary and key secondary endpoints were Assessment of SpondyloArthritis international Society ≥20% improvement (ASAS20) and ≥40% improvement (ASAS40) responses, respectively, at week 16. Safety was assessed throughout., Results: 269 patients were randomised and treated: tofacitinib, n=133; placebo, n=136. At week 16, the ASAS20 response rate was significantly (p<0.0001) greater with tofacitinib (56.4%, 75 of 133) versus placebo (29.4%, 40 of 136), and the ASAS40 response rate was significantly (p<0.0001) greater with tofacitinib (40.6%, 54 of 133) versus placebo (12.5%, 17 of 136). Up to week 16, with tofacitinib and placebo, respectively, 73 of 133 (54.9%) and 70 of 136 (51.5%) patients had adverse events; 2 of 133 (1.5%) and 1 of 136 (0.7%) had serious adverse events. Up to week 48, with tofacitinib, 3 of 133 (2.3%) patients had adjudicated hepatic events, 3 of 133 (2.3%) had non-serious herpes zoster, and 1 of 133 (0.8%) had a serious infection; with placebo→tofacitinib, 2 (1.5%) patients had non-serious herpes zoster. There were no deaths, malignancies, major adverse cardiovascular events, thromboembolic events or opportunistic infections., Conclusions: In adults with active AS, tofacitinib demonstrated significantly greater efficacy versus placebo. No new potential safety risks were identified., Trial Registration Number: NCT03502616., Competing Interests: Competing interests: AD has received grant/research support from AbbVie, Boehringer Ingelheim, Eli Lilly, GSK, Novartis, Pfizer Inc and UCB, and is a consultant for AbbVie, Amgen, Boehringer Ingelheim, BMS, Celgene, Eli Lilly, Galapagos, Gilead Sciences, GSK, Janssen, Novartis, Pfizer Inc and UCB. PS-S and HX have no competing interests to declare. XB is a consultant for AbbVie, Amgen, BMS, Celgene, Eli Lilly, Gilead Sciences, Janssen, MSD, Novartis, Pfizer Inc and UCB. LSG has received grant/research support from Pfizer Inc and UCB, and is a consultant for AbbVie, Eli Lilly, Gilead Sciences, GSK, Novartis and UCB. DF, LW, JW, SM, CW, OD, LF and KSK are employees and shareholders of Pfizer Inc. DvdH is a consultant for AbbVie, Amgen, Astellas, AstraZeneca, Bayer, BMS, Boehringer Ingelheim, Celgene, Cyxone, Daiichi, Eli Lilly, Galapagos, Gilead Sciences, GSK, Janssen, Merck, Novartis, Pfizer Inc, Regeneron, Roche, Sanofi, Takeda and UCB, and is Director of Imaging Rheumatology BV., (© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)