The aim of this investigation was to study the topographic distribution of Actinobacillus actinomycetemcomitans in patients with adult periodontitis before and after mechanical periodontal treatment (repeated oral hygiene instructions, systematic deep scaling, and root planing). In 10 A. actinomycetemcomitans-positive patients, subgingival microbial samples were obtained from the mesial and distal aspect of every tooth (38 to 56 sites per patient, 479 sites in total) before and one month after treatment. The samples were cultured on TSBV agar. A. actinomycetemcomitans was identified based on phenotypical and serological criteria. A. actinomycetemcomitans was present in 40% of the samples taken before and in 23% of the samples taken after treatment. Before treatment, the frequency of A. actinomycetemcomitans-positive samples per patient was wide spread and ranged from 7 to 90%. After treatment, two patterns of A. actinomycetemcomitans distribution could be recognized: the majority of the patients showed only a limited percentage of positive samples and yielded less than 10(5) A. actinomycetemcomitans. In three subjects, however, relatively high numbers of positive sites were still present, and many of these positive sites showed high A. actinomycetemcomitans counts. Logistic multiple regression showed the presence of A. actinomycetemcomitans before treatment depended strongly on the individual and was significantly associated with probing depth (P < 0.001) and bleeding upon sampling (P = 0.07). The highest chance of detecting A. actinomycetemcomitans existed in deep pockets which bled upon sampling. After treatment, there was a strong individual influence and an influence of probing depth (P < 0.001). The highest chance of detecting A. actinomycetemcomitans existed in residual pockets in the range of 5 mm.