Periodontal disease is a multiple factor disease caused by genetic factors, environmental factors, and periodontal bacteria (periodontal pathogens). The present study aimed to elucidate the risk factors for periodontal disease in Japanese adolescents. Subjects (11-16 years old) were classified into three groups: localized aggressive periodontitis (LAP), periodontal attachment loss (PAL), and periodontally healthy (PH) groups. Genomic DNA isolated from the buccal mucosa was used for single-nucleotide polymorphism analyses of the candidate genes (interleukin-1alpha-889; interleukin-1alpha +4845; interleukin-1beta +3954; an immunoglobulin G Fc gamma receptor, FcgammaRIIa-R/H131; and a human leukocyte antigen class II allele, HLA-DQB1) of aggressive periodontitis. Subgingival plaque samples obtained from the same subjects were used for 16S rRNAbased polymerase chain reaction analysis of five important periodontal pathogens (Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, Treponema denticola, and Tannerella forsythia). Tannerella forsythia was detected in the deepest periodontal pockets in all subjects in the LAP and PAL groups. The prevalence of an atypical BamHI restriction site in HLA-DQB1 of the LAP group was significantly higher than that in the PH and PAL groups. Furthermore, all subjects who had the atypical BamHI restriction site in HLA-DQB1 had T. forsythia infection. These results suggested that T. forsythia is associated with periodontal disease in Japanese adolescents and also suggested that HLA-DQB1 is related to LAP and is associated with T. forsythia infection.