BACKGROUND: Patients with inflammatory joint diseases tend due to new treatments to be more physically active; something not taken into account by currently used outcome measures. The Rheumatoid and Arthritis Outcome Score (RAOS) is an adaptation of the Knee injury and Osteoarthritis Outcome Score (KOOS) and evaluates functional limitations of importance to physically active people with inflammatory joint diseases and problems from the lower extremities. The aim of the study was to test the RAOS for validity, reliability and responsiveness. METHODS: 119 in-patients with inflammatory joint disease (51% RA) admitted to multidisciplinary care, mean age 56 (+/-13), 73% women, mean disease duration 18 (+/-14) yr were consecutively enrolled. They all received the RAOS, the SF-36, the HAQ and four subscales of the AIMS2 twice during their stay for test of validity and responsiveness. Test-retest reliability of the RAOS questionnaire was calculated on 52 patients using the first or second administration and an additional mailed questionnaire. RESULTS: The RAOS met set criteria of reliability and validity. The random intraclass correlation coefficient (ICC 2,1) for the five subscales ranged from 0.76 to 0.92, indicating that individual comparisons were possible except for the subscale Sport and Recreation Function. Inter-item correlation measured by Cronbach's alpha ranged from 0.78 to 0.95. When measuring construct validity the highest correlations occurred between subscales intended to measure similar constructs. Change over time (24 (+/- 7) days) due to multidisciplinary care was significant for all subscales (p < 0.001). The effect sizes ranged from 0.30-0.44 and were considered small to medium. All the RAOS subscales were more responsive than the HAQ. Some of the SF-36 subscales and the AIMS2 subscales were more responsive than the RAOS subscales. CONCLUSION: It is possible to adapt already existing outcome measures to assess other groups with musculoskeletal difficulties in the lower extremity. The RAOS is a reliable, valid and responsive outcome instrument for assessment of multidisciplinary care. To fully validate the RAOS further studies are needed in other populations.