The diagnosis of ankylosing spondylitis is mainly based on a radiograph of the sacroiliac joints. Thus, this is the standard imaging test. However, in the early phase of disease, conventional radiographs are often too insensitive to show sacroiliitis. In this clinical situation, HLA B27 testing and new imaging modalities such as magnetic resonance imaging may be helpful. Early forms of AS can be initially classified as undifferentiated spondyloarthritis. All subsets of spondyloarthritides may evolve later into AS. Since AS in association with psoriasis and chronic inflammatory bowel diseases is often HLA B27-negative, this test is of limited value under these circumstances. The usefulness of testing for HLA B27 and its subtypes differs among ethnic and racial groups. The value of this test for diagnosis depends on the individual pre-test probability in each setting.