3 studies investigated the importance of Actinobacillus actinomycetemcomitans, Bacteroides gingivalis and Bacteroides intermedius in progressive adult periodontitis. A total of 196 adults with advanced periodontitis and 475 periodontal sites were examined. The periodontal disease activity was assessed by changes in probing periodontal attachment level and crestal alveolar bone level. The composition of the subgingival microflora was determined by nonselective and selective culturing and indirect immunofluorescence of direct smears. A. actinomycetemcomitans occurred in 50% of progressing lesions and in only 6% of nonprogressing sites. B. gingivalis was detected in 42-52% of progressing lesions and in 14% of nonprogressing sites. The median B. gingivalis recovery in culture-positive sites was more than 10-fold higher in progressing than in nonprogressing sites. B. intermedius was recovered from 59-89% of progressing lesions and from 36-53% of nonprogressing sites. B. intermedius averaged 5-10 higher recovery in infected progressing lesions than in infected nonprogressing sites. 1 progressing lesion only failed to reveal any of the above organisms. Assessment of the overall results indicates that A. actinomycetemcomitans, B. gingivalis and B. intermedius are major suspected pathogens in destructive periodontal disease in adults.