This report documents the results of a study performed to examine clinical use of mizoribine (MZR), using data from a large-scale prospective cohort study, IORRA (Institute of Rheumatology Rheumatoid Arthritis). The number of patients with RA entered in this study from October 2000 through October 2003 was 6238. Three hundred and six patients (4.9%) received MZR therapy. Mizoribine users who were taking methotrexate (MTX) (MTX-MZR group, n = 94) and over 70 years of age (elderly group, n = 45) were collected. Cumulative retention rates of MZR were calculated by Kaplan-Meier analysis. Median drug survival of MZR was 28 months for the poor responders to MTX and 43 months for the poor responders to MZR, with no significant difference between these groups. Cumulative retention rate of MZR in the elderly group did not show a significant difference compared to that in patients aged under 70 years. Ten patients (10.6%) in the MTX-MZR group and 10 patients (22.2%) in the elderly group experienced adverse effects of MZR. None of these adverse effects was serious. This study indicated that, although MZR has not been frequently prescribed for RA patients, it may be useful and relatively safe for patients who are poor responders to MTX as an additional regimen to MTX therapy as well as for elderly patients.