Background: The Gram-negative facultatively anaerobic coccobacillus Actinobacillus actinomycetemcomitans is the major pathogen in localized juvenile periodontitis (WP) and some forms of adult periodontitis (AP). A. actinomycetemcomitans can be grouped into 5 serotypes (a through e) based on differences in the carbohydrate moiety of cell surface lipopolysaccharide. The A. actinomycetemcomitans population is genetically heterogeneous. Since the studies on A. actinomycetemcomitans colonization have mostly applied only culture techniques, the clonality of the follow-up isolates has not been established Thus, it is possible that, although A. actinomycetemcomitans could be repeatedly isolated from-an individual, the intitial colonizing strain was replaced by another strain. The aim of the study was to determine whether oral A. actinomycetemcomitans strains change spontaneously over time or after periodontal treatment.