The purpose of this report is to illustrate the use of microbiology in the diagnosis and treatment in four cases of severe adult periodontitis. All four patients were subgingivally infected with Actinobacillus actinomycetemcomitans which was the basis for an adjunct minocycline therapy. Scaling and root planing plus minocycline were not able to establish a clinically stable periodontal condition. A. actinomycetemcomitans was not eliminated from the pockets in any of the patients. Good clinical and microbiological results were obtained after continuous conventional treatment with an adjunct therapy of metronidazole plus amoxycillin. With a 7 day course of this antibiotic therapy, the pockets of all four patients became free of A. actinomycetemcomitans and were still negative on repeated sampling on recall visits. Microbiological monitoring of severe periodontitis in adults seems to be a valuable adjunct to clinical examination. Furthermore, effective antibiotic therapy in A. actinomycetemcomitans-associated periodontitis appears of paramount importance in obtaining a stable periodontal situation.