Abstract
Ankylosing spondylitis (AS) is a chronic inflammatory disease of the axial bones, primarily affecting the spine. AS is classified as spondyloarthritis because it affects the spine and other joints. AS has several presentations and, in some cases, can be insidious, making it difficult to diagnose. We encountered a patient on long-term follow-up for rheumatoid arthritis with an appearance of AS. This case suggests that patients with long-term rheumatoid arthritis can develop AS during follow-up and that the seropositivity of rheumatoid factors and anti-cyclic citrullinated peptide antibodies cannot rule out AS. Therefore, the possibility of AS should be considered, even in patients diagnosed with rheumatoid arthritis.