Periodontal disease is a complication of patients with type 1 diabetes mellitus (T1DM), although the mechanisms responsible for this relationship remain unclear. The aim of this study was to examine oral manifestations and the prevalence of periodontal pathogens from subgingival plaque samples and serum IgG antibody levels against them in young Japanese type 1 diabetic subjects. One hundred and seventeen Japanese T1DM subjects (53 male, 64 female, mean age +/- SD, 16 +/- 6.5 years) participated in this study. Thirty-nine periodontally healthy, age-matched nondiabetics served as controls. T1DM subjects were clinically assigned into three groups: 12 periodontitis, 32 gingivitis and 73 periodontally healthy. Microbiological tests for four periodontal pathogens, Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, Prevotella intermedia and Capnocytophaga ochracea were performed using 16S ribosomal RNA-based polymerase chain reaction methods. Serum IgG antibody levels against 12 periodontal bacteria including the four species assessed by polymerase chain reaction were measured by enzyme-linked immunosorbent assay. In the T1DM subjects, the Periodontitis group had a significantly longer mean duration of diabetes and a higher percentages of subjects harbouring P. gingivalis and P. intermedia than the Periodontally Healthy group. Serum IgG antibody levels against P. gingivalis were significantly elevated in the Periodontitis group compared with Gingivitis and Periodontally Healthy groups. These results indicate that Japanese T1DM subjects are a high-risk group for periodontal disease and both P. gingivalis infection and duration of T1DM are risk factors for the progression of periodontitis in patients with T1DM.