Ankylosing spondylitis is the prototype of an interrelated group of disorders termed spondyloarthropathies, which include reactive arthritis, psoriatic arthritis, and rheumatic disorders associated with inflammatory bowel disease. It can be difficult to differentiate between these disorders because they may occur simultaneously or sequentially. In addition, some of the clinical characteristics of these diseases, such as enthesiopathy and eye involvement, are similar no matter what the diagnosis. The monitoring, diagnosis, and treatment of these diseases are related more to their clinical presentation than to the precise diagnosis.