Treatment of patients suffering from A. actinomycetemcomitans (A. a.)-associated periodontitis aims to eliminate this microorganism. Recolonisation or persistance of A. a. after therapy has been considered as a risk factor for recurrent periodontitis.The purpose of this study was to evaluate the long-term microbiological and clinical effect of a combined mechanical and antibiotic treatment in A. a.-associated periodontitis at least 12 months (mean 36,8 months) after active therapy. This retrospective study included 40 patients (28 female) with a mean age of 37 years, who received mechanical subgingival debridement supplemented with an antibiotic therapy (metronidazole plus amoxicillin/ciprofloxacin). The presence of A. a. after treatment was determined using two commercial nucleic acid probes (IAI PadoTest, DMDx) and culture technique (TSBV). The clinical situation was assessed according to the probing depths.Microbiological reexamination revealed that A. a. was undetectable in 32 patients. The IAI PadoTest showed a positive result for A. a. in 7 and the DMDx in 2 cases. In only one patient A. a. could be detected with both nucleid acid probes and the culture. Clinical improvement was observed in all patients directly after therapy, resulting in a significant reduction of the pocket depths. The clinical outcome was stable over the observation period with no significant difference between patients with or without a postive evidence for A. a. In conclusion, a combined mechanical and antibiotic therapy followed by a regular maintenance is effective to suppress A. a. over a mean period of 37 months, is appropriate for improving the clinical situation, and achieving long-term stability of periodontal health. Schlagwörter: actinobacillus actinomycetemcomitans, microbiological diagnostic