© 2016 The Authors. Arthritis & Rheumatology published by Wiley Periodicals, Inc. on behalf of the American College of Rheumatology. Objective: To determine the proportion of patients with axial spondyloarthritis (SpA) among those with chronic back pain and ≥1 of 3 SpA features in the US. Methods: The study was conducted at rheumatology practices in the US. Patients were required to have chronic back pain for ≥3 months beginning at <45 years of age, no prior SpA diagnosis, and ≥1 of the following 3 SpA features: HLA–B27 positivity, current inflammatory back pain, and magnetic resonance imaging (MRI) or radiographic evidence of sacroiliitis. Medical history and physical examination findings, pelvic radiographs, MRIs of sacroiliac joints, C-reactive protein (CRP) level, and HLA–B27 status were obtained. Investigators were asked if a clinical diagnosis of axial SpA could be made based on the results. Data were also analyzed separately to determine if patients fulfilled the Assessment of SpondyloArthritis international Society (ASAS) criteria for axial SpA and/or modified New York criteria for ankylosing spondylitis (AS). Results: A total of 751 patients were enrolled (46% were existing patients in rheumatology practices, 40% were new referrals, and 14% were self referred). Among patients with available data, 319 of 697 (46%) were diagnosed as having axial SpA by the investigator, and 348 of 744 (47%) fulfilled the ASAS criteria, of whom 238 were classified as having nonradiographic axial SpA and 108 as having AS; 2 had missing data. Using investigator's clinical diagnosis as the gold standard, the specificity and sensitivity of the ASAS criteria were 79% and 81%, respectively. Conclusion: Our findings indicate that among patients with chronic back pain for ≥3 months beginning at ages younger than 45 years, the presence of ≥1 of 3 SpA features is an effective way to identify those with possible axial SpA.