A total of 235 subgingival sites, including 104 progressive deep lesions from 61 untreated patients, 26 progressive deep lesions from 10 treated patients, 33 nonprogressive deep sites from 20 untreated patients, and 72 nonprogressive sites from 55 treated patients were examined for Actinobacillus actinomycetemcomitans, Bacteroides gingivalis and Bacteroides intermedius. The periodontal disease progression was mainly determined on the basis of radiographic changes in the crestal alveolar bone level. A. actinomycetemcomitans isolation was carried out using the selective TSBV medium and B. gingivalis and B. intermedius isolations were performed using a nonselective blood agar medium. 1 or more of the 3 bacteria studied appeared in 99.2% of progressive periodontal lesions but only in 40.0% of nonprogressive sites. Culture-positive progressive periodontal sites in comparison with culture-positive nonprogressive sites showed higher median recovery rates of A. actinomycetemcomitans (0.5% vs 0.3%), B. gingivalis (30.5% vs 0.3%) and B. intermedius (4.9% vs 0.5%). Of total progressive lesions, 12.3% yielded solely A. actinomycetemcomitans, 21.5% demonstrated solely B. gingivalis, and 20.8% revealed solely B. intermedius. The A. actinomycetemcomitans--B. intermedius combination was found in 24.6% of progressive lesions. A. actinomycetemcomitans appeared in significantly higher prevalence in treated-progressive lesions (80.8%) than in nontreated-progressive lesions (42.3%). 32 of the 42 culture-positive nonprogressive sites yielded B. intermedius as the sole test organism. The main conclusion is that A. actinomycetemcomitans, B. gingivalis and B. intermedius are closely related to disease-active periodontitis, and more closely than to periodontal pocket depth. This finding is important in understanding periodontal disease etiology and pathogenesis and may also aid in a clinical setting to differentiate progressing and nonprogressing periodontal sites.