OBJECTIVES: QOL (quality of life) is critical in the management of patients with rheumatoid arthritis (RA). To evaluate the QOL of RA patients with different background, we evaluated the QOL by using Japanese version of EuroQol (EQ-5D) in a large observational cohort study of Japanese RA patients, IORRA.     METHODS: We have established a large observational cohort of RA patients IORRA (Institute Of Rheumatology Rheumatoid Arthritis) in the Institute of Rheumatology Tokyo Women's Medical University since 2000. Essentially all RA patients who consulted there were registered, and clinical parameters including the disease activity, use of drugs and the occurrence of adverse events in daily clinical settings were assessed biannually based on patient's report, physician's examination and laboratory data. In this cohort, we evaluated the QOL of RA patients by EQ-5D, disease activity by DAS28, and disability by JHAQ, and then we analyzed the related factors for EQ-5D by Spearman's correlation. RESULTS: In September 2007, a total 5023 RA patients (female 84.2%, average 58.02 years-old, average disease duration 11.26 years, rheumatoid factor positive 74.8%, patients taking steroid, methotrexate and biologics were 51.0%, 63.6%, and 4.3%, respectively) fulfilled the questionnaire of EQ-5D. Mean±SD of EQ-5D, DAS28 and JHAQ was 0.757±0.178, 3.28±1.147 and 0.737±0.769, respectively, EQ-5D was worse in female (0.75±0.177) than in male (0.798±0.177) patients, and worse in rheumatoid factor positive (0.75±0.178) than negative (0.782±0.175) patients. EQ-5D became worse by older age and longer disease duration. No clear relationship was identified between EQ-5D and medications including corticosteroid, methotrexate or biologics in this cross-sectional analysis. CONCLUSIONS:   EQ-5D based QOL was analyzed in a large number of Japanese RA patients using IORRA cohort.