Localized juvenile periodontitis (LJP) has been used as a model for studying periodontal disease, and its prevalence is considered to be higher in third-world countries (0.3-8%) than in industrialized countries (0.1%). Mostly, the disease has been associated with Actinobacillus actinomycetemcomitans (A.a.) but lack of association has also been reported. The aim of this study was to identify LJP patients in geographically different Brazilian populations and assess the presence of A.a. in their periodontal lesions. 7843 children, 12-19-years of age, from the cities of Rio de Janeiro, Votorantim and Belo Horizonte were screened, and LJP patients were identified by strict clinical and radiographical criteria. A final LJP prevalence of 0.3%, with a 99% confidence interval between 0.16% to 0.47%, was found. The prevalence in the subpopulations varied between 0.1-1.1% in the different areas. Subgingival bacterial samples were obtained from the oral cavity of 25 patients and their family members. 80% of these patients, 39.5% of their family members, 35.3% of their parents, and 43.9% of all siblings were culture positive for A.a. All but one of the families had at least one member in addition to the patient who was culture positive for A.a. In 3 families, > 1 member showed radiographic and clinical signs of LJP. 30% of non-LJP subjects coming from one of the areas with higher LJP prevalence harbored A.a. We conclude that LJP is highly associated with A.a. in this Brazilian population.