OBJECTIVE To develop a continuous composite index of disease activity for rheumatoid arthritis (RA) based on the 7 American College of Rheumatology (ACR) core data set of disease activity measures: Mean Overall Index for Rheumatoid Arthritis (MOI-RA). METHODS The MOI-RA is the mean of standardized values of tender and swollen joint counts (28, 42, or 66/68 joint counts), physical function (Health Assessment Questionnaire 0-3), patient's and physician's assessments of global health and patient's assessment of pain (visual analog scale 0-100 mm) and erythrocyte sedimentation rate (1-100). All the 7 components were standardized (0-100), and the mean of standardized values was calculated. The range of MOI-RA is 0-100; higher values indicate poorer outcomes. The validity and measurement properties of MOI-RA were analyzed in 169 patients in the Finnish RA Combination therapy trial. RESULTS The mean MOI-RA28 decreased from 38.5 to 13.3 [standardized response mean (SRM) = 1.8, effect size (ES) = 1.9] from baseline to 6 months, compared to Disease Activity Score (DAS) 28, which decreased from 5.55 to 2.77 (SRM = 2.0, ES = 2.8). Correlation between MOI-RA28 and DAS28 was 0.90. When compared to the ACR response categories (20/50/ACR remission), changes in MOI-RA versions (using 28/42/66 joints) were similar. The reproducibility of MOI-RA with different joint counts was 0.97. A simulation in which 15% of the component values of MOI-RA were randomly omitted indicated an intraclass correlation coefficient of 0.98 between incomplete and complete data. CONCLUSIONS MOI-RA is a simple and feasible index based on the ACR core data set of disease activity measures for assessment of disease activity and treatment response in RA trials and clinical settings.