Objective: To examine the validity, reliability, and sensitivity to change of a simple measure of activity participation for rheumatoid arthritis (RA) clinical trials. Joint damage from RA significantly limit patients' participation of daily work and non-work activities, however, few instruments were available to measure treatment effect on this aspect. Methods: We measured activity participation in two randomized clinical trials of abatacept in active RA patients. Activity participation was assessed by two items: 1) the number of days in the past month a patient was unable to perform usual activities (paid or unpaid work, or any other daily activities), and 2) how often a patient was able to perform activities completely (scored 1-6, ranging from none to all of the time). For construct validity, correlations between the two activity items with clinical response and patient-reported outcomes were examined. Intraclass correlation coefficient (ICC) was used to assess test-retest reliability, and standardized effect sizes (SES) were calculated to evaluate sensitivity to change. Results: In both studies at baseline, patients were limited for 15 days per month in usual activities and the score on activity completion was 3.7. After treatment, patients with EULAR clinical responses of good, moderate, none, gained back 11, 9, and 4 days of activity respectively, and patients who achieved minimal disease activity state gained 12 days vs. those who did not (7 days). Similar pattern was observed for the activity completion score. Moderate to strong correlations (0.5-0.6) between the two activity items with physical function, patient global, pain, and fatigue were found. The ICC for reliability was 0.6, and the SES was 0.5, indicating good response to change. Conclusion: The simple activity participation measure reflects true changes in patient clinical status and quality of life. It is valid, reliable, and sensitive to change, which suggests it is a suitable outcomes measure for clinical trials.