The Ankylosing Spondylitis Disease Activity Score (ASDAS) is a new composite index to assess disease activity in ankylosing spondylitis (AS). Criteria for disease activity states and improvement scores are important for use in clinical practice, observational studies, and clinical trials, and have been proposed by the Assessment of SpondyloArthritis international Society (ASAS). At OMERACT 10, our aim was to obtain endorsement from OMERACT for the ASDAS disease activity states and response criteria proposed by the ASAS membership. This article summarizes the associated discussions, the scientific basis of the validation process, and the results from the voting sessions, and identifies areas for research using the ASDAS. OMERACT participants agreed with the selection of cutoff values, which now have the combined endorsement of ASAS and OMERACT and are ready to be used in clinical practice and in research settings.