In this study, the prevalence and serotype distribution of Porphyromonas gingivalis in an Indonesian population (n=158) is described. The relationship between subgingival P. gingivalis and periodontal attachment loss was investigated. The serotype distribution and periodontal parameters were studied. Serotyping was also used to study person-to-person transmission between siblings and between spouses. Approximately 50% of the subjects had periodontal attachment loss > or =3 mm at 1 or more recorded sites. The population was divided into 2 subgroups based on the presence or absence of P. gingivalis. No differences in plaque index, pocket depth, clinical attachment loss, bleeding upon probing, % of sites with > or =3 mm clinical attachment loss and % of sites with probing pocket depth > or =5 mm, were observed between both sub-populations. All known 6 capsular serotypes were found in the study population, with the exception of the K1 type. Detection of 1 of the known P. gingivalis serotypes was not related with the amount of clinical attachment loss. In 3 out of 29 sibships with more than one member positive for P. gingivalis, an identical P. gingivalis serotype was found. None of the 15 couples in the study shared an identical P. gingivalis serotype, indicating that transmission is probably not a common phenomenon in this population.