In 15 adult patients with severely advanced periodontal disease unrelated to localized juvenile periodontitis high numbers of A. actinomycetemcomitans (A. a.) were identified in samples of subgingival plaque (median log CFU 5.13). In order to correlate clinical conditions with bacteriological findings, in every patient 2 deep periodontal pockets were treated with a 3-phase regimen of minocycline administration, mechanical, and surgical periodontal therapy. A. a. was eliminated by minocycline alone in 2 cases. Minocycline in combination with subgingival scaling resulted in 6 patients in A. a. negative samples. Following termination of minocycline therapy, frequently increasing numbers of A. a. were observed. Patients showed significant gain in clinical attachment or reduction of probing pocket depth during different phases of therapy, but only if A. a. was suppressed close to or below the lower limit of detection (5 CFU/ml). Considerable problems with elimination of A.a. may be connected with frequently observed recurrent periodontal disease in these patients.