OBJECTIVE: To examine the validity, reliability and sensitivity to change of the Activity Participation Questionnaire (APaQ), a simple measure of activity participation for patients with RA. METHODS: The questionnaire contained two items: (i) number of days in the past month of being unable to perform one's usual activities because of RA; and (ii) a score measuring how often one's usual activities could be completed. The APaQ was administered to 1043 RA patients in two clinical trials of abatacept. Construct validity was evaluated by examining changes from baseline in activity scores by clinical response measured by the European League Against Rheumatism (EULAR) and ACR criteria and minimal disease activity (MDA) state and by correlations with patient-reported outcome measures of physical function, disease activity, pain and fatigue at study end-point. Internal consistency, test-retest reliability and sensitivity to change were assessed. RESULTS: Both activity participation items were significantly associated with levels of EULAR and ACR response and the achievement of MDA state (P < 0.0005 for all comparisons). Moderate correlations with patient-reported outcomes were consistently found (correlations 0.5-0.6). Cronbach's alpha was 0.7 indicating good internal consistency, the intraclass correlation coefficient of 0.6 suggesting acceptable test-retest reliability. Sensitivity to change was demonstrated by the treatment differences and the standardized response mean (0.39 and 0.30) for the two activity items. CONCLUSION: The APaQ is a simple, reliable and valid measure of patient activity, which is sensitive to change, suggesting its suitability for use in clinical trials.