OBJECTIVES: To maintain good quality of life (QOL) is an important goal in the management of patients with rheumatoid arthritis (RA). Since QOL of RA patients is influenced by many factors, we quantitatively measured QOL of RA patients based on Japanese version of EuroQoL (EQ-5D), and we analyzed the major determinants of EQ-5D in RA patients using a large observational cohort of RA patients in Japan. METHODS: IORRA (Institute Of Rheumatology Rheumatoid Arthritis) is an observational cohort of RA patients established 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 QOL of RA patients using EQ-5D, and then analyzed the factors that influenced on EQ-5D by the analysis of variance. The factors analyzed here were gender, age, disease duration, disease activity, disability, and comorbidity. RESULTS: In September 2007, a total 5,023 RA patients (female 84.2%, average 58.0 years-old, average disease duration 11.3 years, rheumatoid factor positive 74.8%, patients taking steroid, methotrexate and biologics were 51.0%, 63.6%, and 4.3%, respectively) answered the questionnaire of EQ-5D. The factors aand the percentage of their contributions on EQ-5D were as follows; i) disability assessed by JHAQ (Japanese Health Assessment Questionnaire): 57.6%, pain VAS (visual analog scale): 12.5%, female: 6.1%, joint surgery: 5.9%, non-orthopedic surgery: 5.4%, duration of disease: 1.8% and occurrence of fractures: 1.3%. CONCLUSIONS: QOL of RA patients is deeply influenced by the disability together with comorbidity. Management for the control of both disease activity and comorbidity is crucial.