Objective.Anticitrullinated protein antibodies (ACPA) are relatively specific for rheumatoid arthritis (RA), and predate disease. The oral pathogenPorphyromonas gingivalismay play a role in breaking immune tolerance to citrullinated antigens. We studied a cohort of patients with RA and their relatives looking for associations between anti-P. gingivalisantibodies and ACPA.Methods.Patients with RA (n = 82) and their relatives (n = 205) from a North American Native (NAN) population were studied, along with 47 NAN and 60 non-NAN controls. IgM and IgA rheumatoid factor (RF) were tested by nephelometry and ELISA. Second-generation anticyclic citrullinated peptide (anti-CCP2) isotypes and IgG anti-P. gingivalislipopolysaccharides were tested by ELISA. HLA-DRB1 typing was performed by sequencing. Oral hygiene and smoking habits were assessed by questionnaires.Results.Autoantibody frequency in patients with RA and relatives: ACPA 91% vs 19%, respectively; IgM RF 82% vs 17%; IgA RF 48% vs 22%. Anti-P. gingivalislevels were higher in patients with RA compared to relatives and controls (p = 0.005) and higher in ACPA-positive patients with RA than in ACPA-negative patients with RA (p = 0.04) and relatives (p < 0.001), but comparable in RF-positive and RF-negative patients and relatives. Poor oral hygiene and smoking were prevalent, but with no clear association with autoantibodies. Relatives with 2 shared-epitope alleles were more likely to be ACPA-positive (OR 2.5, p = 0.02).Conclusion.In a genetically predisposed population of NAN patients with RA and their relatives, anti-P. gingivalisantibodies were associated with ACPA. These findings suggest that immune responses toP. gingivalismay be involved in breaking immune tolerance to citrullinated antigens.