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No study available has utilized the new classification scheme (the consensus report of the American Academy of Periodontology 1999) to determine the prevalence of Actinobacillus actinomycetemcomitans in different periodontal conditions. The purpose of this study was to investigate prevalence and proportions of A. actinomycetemcomitans serotypes in subgingival plaque samples from a young Taiwanese population with aggressive periodontitis, chronic periodontitis and no periodontal disease. A total of 221 subgingival plaque samples from 171 diseased subjects (70 had aggressive periodontitis, and 101 had chronic periodontitis) (mean age 25.0 ± 8.2 yr) and 50 periodontally healthy subjects (mean age 18.4 ± 9.5 yr) were screened for A. actinomycetemcomitans. Serotypes of A. actinomycetemcomitans were determined by an indirect immunofluorescence assay using serotype-specific polyclonal antisera to A. actinomycetemcomitans strains ATCC 29523 (serotype a), ATCC 43728 (serotype b) and ATCC 33384 (serotype c). Prevalence (% of positive samples) of A. actinomycetemcomitans was 84.3% in aggressive periodontitis, 60.4% in chronic periodontitis, and 64.0% in periodontally healthy subjects. Proportions of A. actinomycetemcomitans (mean percentage per total bacteria) in periodontally healthy subjects were significantly lower than in aggressive periodontitis subjects. The proportion of serotype b in subjects with aggressive periodontitis and subjects with chronic periodontitis were significantly greater than that in periodontally healthy subjects. The proportion of serotype c in periodontally healthy subjects was much higher than that in chronic periodontitis subjects. The results of this study suggest that prevalence and proportions of A. actinomycetemcomitans are significantly greater in patients with aggressive periodontitis than in those with chronic periodontitis. Serotype b is the predominant serotype of A. actinomycetemcomitans in patients with diseased periodontal conditions. Serotype c is a more common serotype detected in periodontally healthy subjects.