Objective: Actinobacillus actinomycetemcomitans is a major periodontal pathogen. This research was aimed at investigating the prevalence of A actinomycetemcomitans in Chinese chronic periodontitis patients and periodontally healthy adults. Method and Materials: A total of 116 chronic periodontitis patients and 111 periodontally healthy adults were included. In each periodontitis patient, subgingival plaque samples were collected from 2 molar sites with the deepest pockets and 1 periodontally healthy site. The samples of periodontally healthy adults were obtained from the mesiobuccal site of 1 maxillary first molar. A actinomycetemcomitans was detected by 16S rRNA polymerase chain action. Pocket depth, clinical attachment loss, and bleeding on probing of the sampled sites were recorded. Results: A actinomycetemcomitans was detected in only 1 site (0.90%) of periodontally healthy subjects. For chronic periodontitis patients, the prevalence of periodontitis sites (33.62%) was significantly higher than that of healthy sites (0.90%) (P < .05); the occurrence showed a decreasing trend as patient age increased: highest in the 20- to 35-year-old group (44.12%), followed by the 36- to 55-year-old group (36.36%) and 56- to 75-year-old group (22.73%) (P < .05). A actinomycetemcomitans was most frequently detected in sites with pocket depth 7 mm or more and clinical attachment loss 6 mm or more (P < .05) and more often detected in sites that exhibited bleeding on probing (37.07%) than those that did not (7.41%) (P < .05). Conclusions: A actinmycetemcomitans was more frequently detected in periodontitis sites than in periodontally healthy sites. For chronic periodontitis patients, a higher prevalence was associated with the severe sites than moderate and mild sites. A actinomycetemcomitans is considered to be a major pathogen in the etiology of chronic periodontitis. (Quintessence Int 2009;40:53–60) Key word: Actinobacillus actinomycetemcomitans, chronic periodontitis, polymerase chain action