A significant association has been shown to exist between periodontitis and rheumatoid arthritis (RA) in observational studies. Some intervention studies have suggested that periodontal treatment can reduce serum inflammatory biomarkers such as C-reactive protein, or erythrocyte sedimentation rate. Serum antibodies to oral bacteria have been found at increased levels in RA patients. In addition, DNAs from these bacteria have been identified from samples of RA patients. Several antibiotics are effective on RA. Finally, oral bacteria are responsible for the etiology of RA.