© Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/. Objective To develop and validate a responsive and feasible ultrasound inflammation score for rheumatoid arthritis (RA). Methods We used data from cohorts of early RA (development) and established RA starting/switching biologic therapy (validation). 4 tendons and 36 joints were examined by a grey scale (GSUS) and power Doppler semiquantitative ultrasound (PDUS) scoring system (full score). Ultrasound score components were selected based on factor analyses of 3-month change in the development cohort. Responsiveness was assessed by standardised response means (SRMs). We assessed the proportion of information retained from the full score by linear regression. Results 118 patients with early and 212 patients with established RA were included. The final ultrasound score included 8 joints (metacarpophalangeal 1-2-3, proximal interphalangeal 2-3, radiocarpal, metatarsophalangeal 2-3) and 1 tendon (extensor carpi ulnaris) examined bilaterally. The 6-month SRMs for the final score were -1.24 (95% CI -1.47 to -1.02) for GSUS, and -1.09 (-1.25 to -0.92) for PDUS in early RA, with 87% of total information retained for GSUS and 90% for PDUS. The new score performed somewhat better than formerly proposed scores in the validation cohort. Conclusions The Ultrasound in Rheumatoid Arthritis 9 joint/tendon score (USRA9) inflammation score showed good responsiveness, retained most of the information from the original full score and overall performed better than previous scores in a validation cohort. Trial registration numbers NCT01205854, ACTRN12610000284066; Post-results.