We report on the microbiological and clinical effectsof mechanical debridement in combination with metronidazole plus amoxicillin therapy in 118 patients withActinobacillus actinomycetemcomitans‐associated periodontitis. Patients were categorized into 3 groups: 28 had localized periodontitis; 50 had generalized periodontitis, and 40 had refractory periodontitis. After initial treatment and metronidazole plus amoxicillin therapy 114 of 118 (96.6%) patients had no detectableA. actinomycetemcomitans.Significant reduction in pocket probing depth and gain of clinical attachment were achieved in almost all patients. Four patients were still positive forA. actinomycetemcomitansafter therapy. Metronidazole resistance (MIC>25 μg/ml) was observed in 2 of 4 strains from these patients. Patients still positive forA. actinomycetemcomitansorPorphyromonas gingivalisshowed a significant higher bleeding tendency after therapy. It was concluded that mechanical periodontal treatment in combination with the metronidazole plus amoxicillin therapy is effective for subgingival suppression ofA. actinomycetemcomitansin patients with severe periodontitis.J Periodontol 1992;63:52–57.