Three hundred and fifty consecutive AS patients fulfilling the modified New York criteria for the classification of AS, and with a new anti-TNF agent prescription (either for the first time or switched) in the last two weeks period will be included. Treatment with anti-TNF agents will be in accordance with the regulations of Turkish Social Security Agency (SGK) on the initiation and continuation of anti-TNF agents in AS, as well as ASAS/EULAR recommendations for the management of AS.