Abstract Background and Aims IgA nephropathy (IgAN) is one of most common primary glomerulonephritis, whose pathogenesis had remained unclear. We had reported that C.rectus and T.denticola, kinds of major periodontal disease bacteria, in tonsils with IgA nephropathy patients were specific to IgAN patients compared with chronic tonsillitis (Nagasawa-Y et al, Plos One, 2014). We also reported C.rectus and S.mutans increased proteinuria synergistically (Misaki-T et al, Nephron, 2018). Red complex of bacteria related with periodontal disease consists of P.gingivalis, T.denticola, and Tannerella forsythia, which has strongest pathogenicity of periodontal disease (Figure 1). But, in this point there were no report of prevalence of red complex of periodontal bacteria in IgA nephropathy. In this study, we evaluated the periodontal disease bacteria including P.gingivalis, T.denticola, and Tannerella forsythia in tonsils of IgAN patients, and the relationship between these periodontal bacteria clinical features in IgAN patients. Method Tonsils were obtained from 23 IgAN patients and 63 chronic tonsillitis patients when the tonsillectomy was operated. mRNAs were extracted from tonsils and the prevalences of P.gingivalis, T.denticola, and Tannerella forsythia were evaluated by RT-PCR using bacteria specific primers. All patients gave the written informed consent which was approved by Hyogo College of medicine. Results Average age was 33+-14 in IgAN patients, and the age in control patients was 27+-7. The average proteinuria in IgAN patietns was 0.9+-1.1g/gcre, and average hematuria was (2+). The prevalence of T.denticola was very low in both groups (0%, 1.6% respectively). The prevalence of P.gingivalis in IgA patients was significantly higher than that in control patients (33% vs 3.2%, respectively, P<0.0001), and the prevalence of Tannerella forsythia in IgAN patients was also significantly higher than that in control (28% vs 6.4%, respectively, P<0.05) which had not been reported (Figure 2). The types of cilia of P.gingivalis (fim A types) were also evaluated. Obviously untypeable of fim A, which is usually minor type, was dominant in IgAN patients. Conclusion Prevalence of red complex of periodontal disease bacteria, especially P.gingivalis, Tannerella forsythia in IgAN patient was higher than that in control. Red complex of periodontal disease bacteria might have some relationship with pathogenesis of IgAN