Abstract: Objective. The objective of this study was to verify the usefulness of a simple disease activity index (SDAI) for rheumatoid arthritis (RA). Methods. The SDAI is the numerical sum of five outcome parameters: tender and swollen joint count (based on a 28-joint assessment), patient and physician global assessment of disease activity [visual analogue scale (VAS) 0-10 cm] and level of C-reactive protein (mg/dl, normal <1 mg/dl). Analysis initially focused on MN301, one of the three phase III clinical trials of leflunomide, in order to assess possible correlations between the SDAI and the Health Assessment Questionnaire (HAQ) and Disease Activity Score 28 (DAS 28). Results were then compared with the other two trials, MN302 and US301. A total of 1839 patients were evaluated. At baseline, 6 and 12 months, the SDAI, DAS 28, American College of Rheumatology (ACR) response criteria and mean HAQ scores were determined for each patient and compared by linear regression for significant correlation. The SDAI was compared qualitatively to the ACR 20% at 3, 6 and 12 months. The index was further validated by comparing the SDAI with survey results obtained from rheumatologists' evaluations of disease activity in test cases. The survey results included defining categorical changes in the SDAI indicating major, minor or no improvement in disease activity in response to treatment. Changes in total Sharp score at 6 and 12 months of treatment were determined for each of these categories of the SDAI and for comparable categories of the DAS 28. Results. The mean SDAI calculated for patients at baseline in study MN301 was 50.06 (range 25.10-96.10) and was, respectively, 50.55 (range 22.10-98.10) and 43.20 (range 12.90-78.20) in studies MN302 and US301. In all three trials, the SDAI was correlated with a high level of statistical significance to the DAS 28 and HAQ scores at baseline, endpoint and change at endpoint. Patients achieving the ACR 20, 50, 70 or 90% response showed proportionate changes in the SDAI. Analysis of surveyed physician responses showed a significant association between the perception of disease activity and the SDAI, as well as changes in the SDAI. Qualitative analysis of radiographic progression at 6 and 12 months for patients showing either major, minor or no improvement of the SDAI showed correspondingly larger increases of the total Sharp score at 12 months. Conclusion. The SDAI is a valid and sensitive assessment of disease activity and treatment response that is comparable with the DAS 28 and ACR response criteria; it is easy to calculate and therefore a viable tool for day-to-day clinical assessment of RA treatment. Overall results indicate that the SDAI has content, criterion and construct validity.