The preventive and therapeutic effects of the Citrus flavonoid hesperidin (HES) on the development of collagen-induced arthritis (CIA), a mouse model of rheumatoid arthritis (RA), were investigated. Mice were administered orally HES three times a week starting from either the onset (day 21) of secondary immunization or on day 31, when the CIA development had reached a plateau. In both cases, treatment with HES resulted in a significant suppression of clinical scores and improvement of histological features. These results suggest that oral administration of HES could be effective for treating human RA patients.