There have been many reports that anti-cyclic citrullinated peptide (anti-CCP) antibodies are useful serological markers of early diagnosis and good predictors of the clinical course for patients with rheumatoid arthritis (RA). Anti-CCP antibodies are produced locally in RA joints, and citrullinated proteins are localized in synovial tissue. In this study, we evaluated the clinical significance of anti-CCP antibodies in the knee joint fluids of the patients with RA. Knee joint fluids were obtained from 61 RA patients (6 males and 55 females, 60.0±9.1 years) and 81 osteoarthritis (OA) patients of (22 males and 59 females, 64.3±13.3 years). Anti-CCP antibodies were measured by ELISA (DIASTAT anti-CCP test). Anti-CCP antibody levels in RA patients were significantly higher than those in OA patients (RA: 106.51±45.44 U/ml, OA: 1.07±0.55 U/ml). In RA patients, the value of anti-CCP in the knee joint fluids varies according to the stage and class grade of the disease. Fifty-nine of 61 (96.7%) patients with RA in the knee joint were positive for anti-CCP antibodies. On the other hand, there was no positive cases in OA joint fluids. Therefore, we concluded that an anti-CCP antibody assay in the knee joint fluids is very useful for diagnosis, and it could be a useful clinical predictive test for RA.