To select specific instruments for each domain of the core set for endpoints in ankylosing spondylitis (AS), we gathered all instruments described in the literature to assess the domains chosen as endpoints in AS and sent them to 43 members of the Assessments in Ankylosing Spondylitis (ASAS) Working Group. The following domains were taken into account: function, pain, spinal mobility, patient global assessment, morning stiffness, peripheral joints and entheses, acute phase reactants, x-ray spine, x-ray hips, fatigue. For each instrument the members were asked to judge if the instrument was feasible and relevant. If an instrument was judged to be not feasible or not relevant by more than 50% of the respondents the instrument was deleted from the list. These data were presented during an ASAS workshop and the final decisions were about which instruments to include in the core set. This process was repeated separately for the settings disease controlling antirheumatic therapy (DC-ART), symptom modifying antirheumatic drugs (SMARD) and physical therapy, and clinical record keeping. The response rate to the questionnaire was 72%. For each domain one or more instruments were selected, except for Entheses and Fatigue. The chosen instruments were similar for the 3 above settings. Core sets of specific instruments were selected for the OMERACT filter test for relevance and feasibility. For all these instruments the remaining aspects of the OMERACT filter (truth and discrimination) should be assessed by literature review and if needed by additional research. It is recommended to use these instruments in all research projects in AS.