Objectives: Only a few studies have attempted to detect differences in microbiologic profiles of patients with chronic periodontitis (CP) and aggressive periodontitis (AgP). The aim of this analysis was to assess if clinical diagnosis or other subject factors showed association with the presence of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis in a cohort of periodontitis patients. Method and Materials: Statistical analysis for association between bacterial detection and clinical diagnosis was performed on a total of 267 consecutive periodontitis cases diagnosed with either CP (n = 183) or AgP (n = 84). All subjects had microbiologic samples collected from the four deepest pockets and analyzed by nested polymerase chain reaction. Results: A actinomycetemcomitans was detected in 54% and 48% of CP and AgP subjects, respectively. A slightly higher detection of P gingivalis was found in CP (67%) compared with AgP (52%) cases. The detection of P gingivalis was associated with older age (P = .002), less disease severity (P = .015), and IL6-1480 genotypes (P = .026), while A actinomycetemcomitans was associated with IL6-1480 genotypes (P = .001). Conclusion: Detection of known periodontopathogenic bacteria is not able to discriminate different forms of periodontitis. Keywords: aggressive, bacteria, chronic, genetics, periodontitis